What's to stop them from just going a generation back and using DDR4 instead of DDR5.

There is no one who can convince me that it makes any noticeable difference anyway. When I was putting together a new/used desktop I specifically looked for DDR4 for precisely that reason and I would take any bet that a performance hit would be measured in numbers too small for any user to even notice.

Constantly needing newer hardware with only fractional improvements is the biggest scam in tech. They took their lesson from Apple and Samsung.