A disk is just an install medium and DRM dongle. How long it takes to install the disk is basically moot because once it installs, it plays from the HDD, or in the case of the PS5, the SSD.
I think what would make the most sense on the PS5, especially if it plans to have an internal SSD drive, is a sort of multi-drive setup......that is, use standard (and considerably cheaper) mechanical HHD's to store games, and just load them to the SSD for the faster load times when you run a game. Every time you play a game, it loads it onto the SSD for the zippity-do-da load times while playing. It might even hold the game there until you boot a different game, in which case it loads the new game onto the SSD. While the initial boot might be a bit longer due the the transfer between drives, the payoff would be worth it if it truly is 19X faster than current SSD tech on PC's while playing.
I'm not sure what we should consider to be the average file size for later-in-the-gen PS5 games, but it could be a way of them reducing costs by having only a....say.....250-300GB SSD drive for the games to run from, while the older style HDD simply stores all your games.
Does that sound like a feasible idea? I mean, it sounds logical to me and would be a lot more accessible for the average gamer, especially when adding extended storage.