Well that is bound to happen to a certain extent, but I do not see it as the main contributing factor to Xbox game sales being down, or anything close to it. In fact the games being open to a new audience would only help. They just have to make a compelling product. Forza Horizon 5 is a successful game and that released on both platforms day one, because it's a bloody good game.
Same as Xbox, because PC gamers for the most part, play very different games to console gamers. It's only really console focused places like Gaf where console ports get this big hype on PC, most of my PC friends are playing some MP title on steam I personally don't give a shit about, but those are the hugely popular on PC. I think PC gamers that came from console backgrounds aren't your typical PC gamers, just look at the most popular games on Steam, it's shit Gaf would find niche but are actually huge.