I think that IQ has improved in two main respects. First is resolution, which can be hard to see on the internet. My 5D MkII, with around a 22mp sensor was pretty darn good, up to a 50" print. Now, with either my 50mp Canon 5DSR (for sale BTW) or my Sony a7RIII, I can print 72" easily, with no pixelation.
The other BIG improvement is in dynamic range and the related improvement in high-ISO performance. Shooting at ISO 10,000 is now an everyday thing for me, with no worries about noise or loss of detail (yes, there is loss at pixel-level, but it's really manageable):
Handsome Drop-tine Buck IN Velvet by
David Stephens, on Flickr
Or, I can shoot at ISO 50 and get two or three stops more dynamic range than possible with my old 5D2:
Approaching Storm by
David Stephens, on Flickr
In good light, with limited dynamic range, the only improvement is in resolution, which you can't see here on the internet.