This entire argument is disingenuous.
Training, fielding, batting techniques, competitiveness and quality of fast bowling are the biggest areas of development in cricket. I believe there was a jump even between the 20's and the start of the war, and definitely by the 50's. There was probably a jump, all be it a small one between then and the '70's and 80's.
But there are transcendent talents, an aging Sobers being able to produce an innings of a life time vs a rampaging Lillee
And for Baz to wonder how did Miller's bowling load affect his batting take a gander at Sobers overs per match.
With regards to Bradman, there isn't a serious observer who would call him anything that the greatest batsman of all time, but to pretend he wasn't born into a perfect storm is to not want to see it. Less that great attacks (and body line was basically what the '50's, 70's and 80's were like), flatter than imaginable pitches, primarily minnows and one legit opponent and only having to play in 2 countries. The best pace attack of the era being the WI was somewhat of an eyeopener. Imagine batting in the 80s and 90s vs those guys. Viv didn't have to imagine
Finally, with regards to Viv, no one who went through WSC and especially those like Richards and Chappell who thrived, there can be no dispute over the level of cricket, professionalism and talent they faced. Joffra Archer gave Smith issues, give me a break, and unlike the other 3, Smith is an top tier bat.