I don't know if the FFXV will hit 10 million life time sales. The only reason I say that is because the number of games that are coming out is a lot more than what there was years prior. I'd love to be proven wrong though, as I feel a lot of people tend to not talk about older games like they once did.
I would actually beg to differ on a few counts.
1) The total number of games coming out may, in fact, be higher, but if anything, the number of major games have shrunk due to economic considerations. Back in FFVII's day, games could be made on a much shorter time frame, which allowed for major-but-not-blockbuster games like Vagrant Story, Parasite Eve, Brave Fencer Musashi, Xenogears, etc. to be released alongside major FF releases. Squenix's output nowadays looks a lot different than classic Square, and most of the other AAA game companies have followed a similar trend.
2) The number of games coming out seems largely irrelevant to the people who buy a game later on in its lifespan, who mostly do so because they want to experience that particular game. FFVII doesn't really compete with, say, the newest Call of Duty in any meaningful sense, and if FFXV gets a reputation as something unique and worth playing in its own right (which I think it deserves), its own long tail will probably exist under similar conditions.
3) FFXV has one major advantage insofar as its current highest settings are unlikely to look outdated even next generation, and even after that, diminishing returns will ensure that the game never actually ends up looking ugly. Neither a full remake nor even a FFX-style remaster should ever be necessary, and the amount of contextualization needed to appreciate it should be far lower than a game like FFVII (which was limited by its era in everything from art and animation to localization and proper conveyance of characterization).