Not for the users, no!
Who the f cares what G is that if it does what is needed? Replacing is only reasonable for those who want to push through new things (to sell, keep themselves in their position, be paid). Technical barriers have overcome creatively in countless situations in history and we do not need ever increasing bandwith after a satisfactory level reached. Using old spectrums scarcely used should be possible in alternative manner but not f up the life of customers who pay for the whole thing if can be avoided!!
4G and 5G can share to a good extent but it's not worth making every tower do both.
Who says 4G was satisfactory for even the majority of customers? It wasn't satisfactory to me. Sure, useable, but there were (and still are) a number of things that aren't reliable enough that I'd like to do with the wireless networks available today.
No, it's not an artificial barrier.
Cell phone spectrum is very limited. Making everyone switch every 8-9 years is reasonable.
In particular, 5G was available on many phones before 2020 and 6G might start deploying in 2030.