SSD improve programs' loading time, they do nothing for the latency of the UI of those programs.
When win95 arrived and brought the desktop as we know it to the masses it brought with it some latency that has not been reduced since. Subjectively it has increased, it might just be my patiece that goes shorter, so YMMV.
Yes SSD did bring back some of the lost time, and some more, but the programs don't feel as responsive as in "the old times."
I'm not sure what you mean by mainstream, but SSDs were still pretty niche in 2009 and traditional hard drives have remained to be pretty popular until recently due to the lower cost.
> Both my Mac and my Pc from 2005 are way snappier than todays OSX or Win10. Not as fast but has way less latency.
The single most dramatic performance increase I have ever experienced on a computer was with the advent of the SSD.
Load times < 2009 when SSDs became mainstream were atrocious.