You don't need 4090. Game runs very smoothly on 3070. And providing that devs actualy implement RTGI - meaning that game no longer need to process all old rasterized lighting.
As f Witcher remastered - it simply was poorly optimized on launch, And since then performance was improved both by official patches, and by mods.
Much better example is Metro Exodus. Which actually runs better with RTGI. And that's already a two-year-old release. Another good example is Dying Light 2, game runs very well with RTGI.
It runs smoothly on a 3070 currently without ray tracing. You add ray tracing and you drop down to like 10% of that performance.
The games where ray tracing works like Control and Spiderman, there are basically zero textures in the main graphics, very little foliage and such. You want to take an engine with as much detail as an RPG requires and add ray tracing on top, you basically end up running the game like a slideshow.