There were a lot of amazing advancements from 1950 to 1980. After 1990 and autofocus the build quality of lenses started going downhill. The plastic focusing mechanisms negated a lot of the otherwise good optics.
The affordable category of lenses today also don't gave good optics. While there are good modern optics used in higher-end lenses, it's easier to get better used old optics that outperforms shitty modern optics at the same price point.
There are some nice lenses made today but they are extremely costly. A Sony GM 300/2.8 costs $6000. I got my late 70s Canon SSC Fluorite 300/2.8 for $400 and it's tack sharp corner to corner, and considering I paid only $400 for it, I'm not too worried about taking it everyhwere.
A Sony GM 50/1.2 costs $1900. I paid $300 for my Nikkor early 80's 50/1.2. For f/1.2 portraits sharpness isn't the thing you're usually after, and mine gives absolutely spectacular results.
Back in the 50s-80s almost every lens was built like a tank and they are now available used at a fraction of the price of modern equivalents. If you don't give a crap about autofocus (studio portrait, macro, landscape, product photographers all don't need autofocus), it's a very good deal. Plastic started to show up in the 90s and killed everything.
(If you're shooting sports and wildlife, yeah, prioritize autofocus, but that's not what I do.)