But it's not only a weak argument, it's a useless argument. If Bradman averaged 10 less (meaning 90), his feats would mean just as much as they do now and he would still be in front by such a margin that the distinction between eras would be like arguing if white uniforms or colours are better in ODIs.
Every era will have some advantages and some disadvantages, with respect to other eras. That includes the eras of Sobers, Tendulkar, Hutton and, yes, Bradman. It's not so much Bradman that it demeans but everyone else like Hobbs, Hammond, etc. If Bradman -10 is the reality, then the others are no match for the likes of Graeme Smith, Chanderpaul, etc in terms of averages...which seems quite silly.
Again, if someone were to merely mention that there may be a difference in eras...fine. But IIRC Migara once said Bradman would average something like 70 - a 30 point drop. That is nonsensical - or rather, he didn't put up an explanation that could come close to justifying that opinion.