Not really getting you there.
Not having a tier above is simply that.
I know that if there was new top tier then work would be needed but I think it would be constructive and beneficial work such as making the USA competitive with German at 6-7 BR which they could and should easy be.
It have become apparent with the Spallgate saga and the M1 issues that Gaijin uses creative license as and when it likes so to tie up the loose laces at Tier IV should not be that difficult. Italy ,France,UK etc many need some late forties early fifties vehicles but they are still in the meta.
If China and Japan are weak then they are no more weak than they are currently. A bad nation line up is a bad nation line up regardless of a tier split.
I think a clear line to make the transition from WW2 to Cold war would be beneficial in game play, Immersion and also encourage those who are firm WW2 players to cross the line and proceed up the ladder.
There would still be a few heavy and slow tanks in there but many are early variants where later variants are available. T34 or some IS as examples.
I don’t think such a split is beyond the intelligence of Gaijins devs(although with the map issues I am beginning to wonder)
Maybe WT could tie in CAS to weak land trees and make them fair in other ways other than a Tank for tank SP shootout.