This is actually a funny thing. The APS-C Canon 90D has a slightly higher resolution sensor than the full frame 5Dmk4, and significantly higher than the brand new, also full frame, 1DXmk3. Sony’s APS-C bodies are all 24MP, the same as the a7iii and the flagship a9/a9ii.
Other than dedicated high-res models (5DS, a7R, D850/Z7), high-end cameras routinely have much lower pixel density than cheaper cameras, because resolution doesn’t matter that much for most applications, and pros care for a whole bunch of other features that scale poorly with resolution (such as low light performance and buffer drain speed)
Think of your classic Sports Illustrated photographer... those images are not going to run larger than double truck in a print magazine, and 12 MP is plenty for that. Heck, National Geographic's first digital two-page spread was shot with a Nikon D1X.
And news and sports images need to go out FAST... where again a lower MP count actually helps.
And at the same time it was just as likely to result in poorer pictures because of focus shift when stopped down.
That is no longer necessarily true. For example in Nikon world the current 70-200mm f4 is generally thought to have the same quality as the 70-200mm f2.8.
Lens quality in general has gone way up as digital cameras have exceeded the resolution of film. Now companies have to engineer even mid-range lenses to produce very high-quality images since even mid-range DSLRs might have 20MP sensors or more. And on a computer it's easy to zoom in and pixel-peep... and share your disappointment with everyone else if the lens is bad.
For all the legend-making around the Nikon 300mm f2.0, their latest 300mm f2.8 is going to outperform it in every way expect for the number of photons it lets through.