Exactly.
It took about 100 million(?) Playstations and "only" 10 million PS owners bought any one GT game. So, with only 10 million PS5s, how many are actually going to buy GT7? Certainly not 8-10 miiillion. I mean, if we're going by GT franchise history.
155 million PS2s sold and only 15 million GT2s were sold. SO with only 10 million PS5s sold, how many people are going to buy GT7 on PS5 to outsell PS4? PS4 have 115 million units sold. 8 million GTSports were sold. Of course I don't know, but I'd guess not even half that number of people(4million) will be buying GT7 on PS5. That would be amazing if that many people did buy GT7, but again, I can't see it realistically happening.
This isn't the way it works, by the time Gran Turismo came out the PSX was well established and gamers had no shortage of choice what games to buy/play but nothing quite like Gran Turismo, when Gran turismo 2 relased there were more consoles, but more competiion for players attention. When Gran Turismo 3 released, there were far fewer PS2's on the market than PSX's when GT1 released, but GT3 outsold GT1 with ease. Gran Turismo 4 did worse with a playerbase at launch roughly 3 times the size of Gran Turismo 3's.
The fewer games on the market, the less choice the consumers have, the less choice they have the higher a percentage of people will buy a new title (especially a AAA one).
There's no place for an argument where x% of PSX owners bought GT therefore x% of the current PS5 user base will buy GT7 and x% of PS4 players will buy the PS4 version becuase the current PS5 userbase do not have the same choice PSX owners had when GT1 released and the current PS4 userbase has far more games vyying for the players time, money and attention than the PS5 userbase does.
There is a direct correlation between how early in a consoles life a game releases and what percentage of the current user base purchase the game. The later on after the consoles release the consumer base is higher, but so is the competition for sales with other games and for peoples attention. The PS5 version of GT7 will outsell the PS4 version with ease.
You can see it in the chart
@Famine posted, the first game to release does better. That's not becuase it's a better game (debateable on a generation by generation level) but it's because the first game gains more traction within the consumer base at the time it releases.
Then you have those people who bought a console early in life and then stopped using it, or it broke and they never bothered replacing/reparinig it as well as those who have upgraded to a newer console or are planning to do so and have switched off plans to buy more content for their last gen one.
The evidence speaks for itself, the first title on a new console sells best, and games that relase cross gen sell best on the latest hardware despite the smaller playerbase over the previous gen which has the huge playerbase advangate (on paper).