Saturn definitely has a higher attach rate than the other 5th-gen consoles, tho I don't think it was 15.whatever.
My only issue was with the number of 16.7 or whatever the guy said before.
The Sega CD was partially designed with SNES in mind; the earlier prototypes had it functioning more like PC-Engine CD, with little in the way of enhancement hardware. Over time they added more specifications and features to the design, part of which was influenced by knowing certain features the SFC/SNES would likely have like Mode 7 sprite rotation effects.
You can argue that's what they added later, but that misses the point that the Sega CD was announced before any of those decisions were made, so it was already going to exist and the SNES had nothing to do with the hardware that was coming but at best, changes made to the hardware over time before release.
I think when it comes to N64 in Japan you have to keep in mind that there were certain expectations everyone had. To go from ~ 17 million SFCs to barely over 5 million N64s gen-over-gen is a horrible look, there's no other way about it. Meanwhile, Sega went from a distant 3rd in Japan with MegaDrive to a pretty decent 2nd, with a system that they essentially stopped supporting in the region in late 1998 (meanwhile, Nintendo still actively supported the N64 in Japan up through 2001, a full three additional years, until Gamecube's release).
One saw one of the biggest drops gen-over-gen in a territory ever within the industry, the other saw enough of a gain in that same territory to beat the company which had outsold them almost 6:1 just the generation prior. And that's keeping in mind they did that with a system that effectively winded down by late 1998, and the other system had roughly three more years active on the market but still failed to catch them in units sold (for the Japanese region).
AFAIK, Saturn and PS1 sales in Japan were relatively close for the first year or so, although there are some discrepancies because while Sony reported sold-through Sega reported sold-in (to retailers). However the gap didn't really start growing until games like RE and especially FF VII showed up. Games like those, and those in similar vein, that weren't present or plentiful enough on Saturn.
It's true though that VF basically carried Saturn in Japan (as did VF2) and those were the only games for Saturn in the territory that sold as huge volumes, but I think you're overselling Japan's adoption of 3D gaming being what really aided Saturn in Japan early on; unlike the West, Japan actually still cared about 2D games at retail, and the arcade scene was a lot stronger/healthier over there.
It was not a decent second, it was marginally ahead of the N64.
Sony was ahead before FFviii came out, Sega was already seeing lower shipments which you are correct, they were using in place of Sold, so they were actually selling worse than they looked like which was already bad. But to say games like those weren't present on the Saturn doesn't make sense to me, the Saturn was filled with Jrpgs. The N64 had one?
I believe you are overselling the Saturn in Japan and I mean extremely. If the same games on the PSX suddenly weren't selling inn the Saturn, and people would rather buy an N64 that had none of those games on the Saturn, there's only one reason consumers in the earlier years brought a Saturn, and that was because there was excitement for playing 3D arcade games like Virtua Fighter at home. The Playstation proved it had 3D covered, and several popular 2D games in genres that Japanese people would buy, Sega also had these, but a couple years from launch those were no longer incentives to buy the Saturn, but where for the Playstation. The N64 didn't have them, and it's limited library was more attractive to buyers than the same game they brought on the PlayStaton, on the Saturn.
This should immediately make someone ask the question, who were the Japanese gamers that were buying the Saturn in the first place? The only answer is people who were attractive by the excitement of owning 3D arcade games that can finally be played on a home consoles and as you say, they would buy other games with the Saturns higher attach rate, not 16.7 but higher. The consoles sales dropped once Virtua Fighter 2's appeal wore off and you can actually track this yourself buy looking at the software sales once that happened, the software after that game, the best selling title on the Saturn, all dropped like flies and got worse and worse each FY.
It's not a case of the games being bad because many of these games were practically the game as the ports on the PlayStation, the problem was that Sega only had one game series responsible for shifting consoles, both were Virtua Fighter titles. This same exact thing happened in the United States with Sonic and the Genesis but in the opposite way, the Genesis started out slow instead of fast, and the 3p games that were best sellers during those times the sales were high were released later instead of early like the Saturn, but once Sonic sales collapsed so did the consoles sales and the other game software, this happened in Japan with Virtua Fighter.
The real question is why did this happen a second time, even in another country, in almost the exact same way? That's something I've been trying to figure out myself.
What i suspect is that, perhaps Sega when they have a software title that is associated with a rise in a consoles success and sales, they overdue it in marketing that game series using it as the flagship for the ENTIRE console and its library to the point that when the sales of that title end up failing, the flagship isn't there anymore and the consumers are hardwired to associate that with the console being out of favor, seeing less of a reason to buy one. If the game series that was synonymous with the Saturn was on decline than the Japanese consumer may have been led to believe due tot he marketing everything attached to the Saturn is in decline?
For example, in the US Sonic the Hedgehog was pushed more aggressively than anywhere else, it took two years of Sonic before Mortal Kombat came out, the first 3p game to sell a large number of units on the console. But then as Sonic sales dropped while still being marketed as the number one series and reason to own a Genesis, software sales fell. You had some titles selling millions riding off the peak but then you had a situation where a heavily marketed game with extensive media coverage like Vectorman can sell 500,000 units by the end of the year and then die overnight with apparently no more sales to gain from 20 million US households, it makes a person question how many of those 20 million consoles were still being used? You have Sega writing off $60 million of unsold Genesis inventory which creates more questions.
In Japan, Virtua Fighter was Japanese Sonic the Hedgehog. Doing research on the arcade scene in 1993, and the Saturn in 1994 and 1995 in Japan, it was marketed as, if not more aggressively than Sonic was, Sony in contrast was marketing several 2p and 3p games that were in multiple genres. It may be possible that the same thing that happened in the US with the Genesis, happened in Japan with the Saturn. How many of the 4 million Saturns in Japan in 1996 were still being used? The Dreamcasts sudden entrance in Japan resulted in further sales slowdown that ended with Sega clearing out inventory at discounted prices, and a year later, they would take losses on the Dreamcast with a bunch of unsold inventory.
A real head scratcher.