So I guess that so-called decline in popularity for the franchise was a myth then? People have been saying for years now that Final Fantasy isn't the name it once was, but if XV can pull in impressive numbers like this then I'd say it's still an IP that's very much flourishing.
Of course, Remake will blow XV out of the water.
It's about lifetime sales, but FF has had a gentle downwards trend. VII was about 10 million, VIII about 8m, IX only over 5m, X back up to around 8m (before remaster sales), XII down to about 6m, XIII around 6m again (bolstered later somewhat higher by PC). These numbers aren't exact and don't always include the latest rerelease (FF7 passed 11m after the Steam & PS4 releases), but they're close enough. So there has been a general downward decline, but the series seems to be settling around the 6m mark - which is nothing to sniff at. It's a far cry from the heady days of 7 and 8, but they're good sales.
The big thing is that SE's ambition has always been to build another FF7 moment - they want another 10m seller, a game that they can say sold like COD, Skyrim, Bioshock or The Witcher - all 10m+ sellers. That's their big goal, and every time they swing and don't hit it people kick off the doom and gloom, but these games do sell well. However, their ambitions for something even larger do mean that these games probably are more expensive to make than other games that shift similar numbers. Eventually they're going to either have to be successful there or recalibrate how they make FF and make it with a 6-7m audience in mind rather than a 10m+ audience.
The other big thing that's caused this discourse is that the FF13 sequels did exceptionally poorly (each sold half of its predecessor), but I think that gave a false impression - people aren't as interested in sequels, and the trend there would plainly not be continued with 15.
ANYWAY: FF sells well! Square would like it to sell better, like the heyday. In the end I think 15 will probably settle with very similar numbers to 13, which will make them money but not be considered what they wanted. It's a great game though.