Displays have improved, no question about it. That said, I still think the screens on 2009-2012 era iMac are more than good enough even now (for most use cases). They have LED backlights, wide colour range, and many still work very well. That’s the sad part about those iMacs, the screens could easily be used for years more, as a secondary display, but they cannot be because they are tied to what are essentially paper weight machines.
We still use a screen I got 15+ years ago for a machine that is used just for typing documents. That’s how long lived a good display can be. The same couldn’t be said of a 15+ year old iMac, not with modern software anyway.