There's already comfortably more users with a PS5 Pro or greater GPU using Steam than the PS5 Pro will sell.
Ampere projects 13 million units by 2029.
NVIDIA GeForce RTX 4070 2.44%
NVIDIA GeForce RTX 3080 1.93%
NVIDIA GeForce RTX 4070 Ti 1.26%
NVIDIA GeForce RTX 4070 SUPER 1.09%
NVIDIA GeForce RTX 4090 0.96%
NVIDIA GeForce RTX 4080 0.74%
NVIDIA GeForce RTX 3080 Ti 0.71%
NVIDIA GeForce RTX 3090 0.48%
NVIDIA GeForce RTX 4080 SUPER 0.46%
These add up to around 10% of Steam users and I'd say they qualify as PS5 Pro-tier or better, I didn't even bother dipping into the 7900 XTXs etc until we have a more clear picture of the RT perf.
Last time Valve gave monthly users in 2021 they had 132 million, and it certainly hasn't shrunk since.