Will anyone even be able to get games that are 10 years old or so in 10 years? That's how you save money on games.
Generally, yes. This is one area where a physical game might not be available or get extremely rare. If it was a game that did well, then there will be reprints (all those "game of the year" or "classic" editions), but if a game didn't do as well, they often get heavily discounted and then destroyed. Though they usually only get a really small print run.
The question of whether 10 year old games will be cheaper, is a completely different question, however.
Digital games cost basically nothing to keep online, so if there are no licensing issues they generally stay online for ages.
This would also mean the PS6 would be a console I wouldn't need to get first day - because any game I would want to play will be available to purchase later on. There's no need to scramble and find a physical copy that sells out in minutes as they do nowadays - if it's digital, it's forever, so the rush to buy a game on day 1 is basically gone. Might as well wait for a game to go on sale.
It's a policy I apply for PC games - I generally wait until a game gets below $10 with few exceptions. Sure it means I play the game a few years after it comes out, but it works remarkably well enough. I mean, why pay $70 when a few years later I can pay $10. It's also not like I'm hurting for content since Steam has been around for over 20 years now. It's one of the biggest advantages of digital.
I only expect to pay maybe full price for 1 game on each platform every year. Everything else I expect to buy to cost $10. Maybe $20 if I really wanted it.