What has changed, in general, since your 50D
- sensors have more pixels, and ability to shoot at higher ISO with less apparent noise
- feature improvements like Auto ISO functions better (less illogical in selection of ISO vs shutter speed combination
- faster burst shooting
- ability to shoot video
- mirrorless designs from Canon, with added features
- control ring on lenses for third (assignable) control function
- ability to track animal/human eyes to autofocus
- ability to track the photographer eye to determine point of focus (R3)
Cameras like the 40D/50D, 7DII, 1Ds still take excellent photos...later models can take photos in a wider (more adverse) set of conditions, and do so with faster frame rates and with greater automation than the older models, with higher pixel counts, and with less noise in low light, and AF with wider range of success.
There are lots of folks who do not see sufficient benefit in newer cameras than what they already have...bang for buck tradeoff does not make sense (yet) for them, so they stick to what they have. Not everyone needs 20+ fps shooting, AF in light so low you can scarcely see without letting eyes have 20-30 minutes to acclimate first, or eye focus or eye-following AF point selection, etc. Buy when you NEED something that causes your existing stuff to miss shots, or IQ is insufficient for current needs. Mirrorless lenses cost more, and there are fewer in the pool of used market gear. to reduce cost..so unless you want to throw even more money at it (than simply buying a new body) this can be a carefully thought-out financial decision to make.