Some third-party software pretends to restore this functionality, but they do it by repositioning the mouse to simulate a hover, which introduces a delay and doesn't integrate correctly with the animation. Someone wrote a patch that works by disabling SIP and injecting code (https://github.com/briankendall/forceFullDesktopBar), but eventually stopped maintaining it.
A decade later, I doubt anyone at Apple remembers that this bit of user interface used to be good.
I never noticed that behaviour because I only use mission control in full-screen mode. If you swipe up with three (or four) fingers from a full-screen window the previews are visible immediately. I have no idea why we need a different preview for desktop vs full screen however.
The part of this UX that annoys me is the spaces get re-ordered for no apparent reason. I usually have a few IDE windows open and it's tiring to have to double-check the window hasn't moved.