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.