I imagine there's arguments for, and against, this idea, but bear with me.
What if...
First TR raises your cap to lev 21, cap stone abilities remain as lev 20 req, with the added bonus that you could splash 9 levels of something and still get second tier PrE's.
Second TR raises your cap to lev 22, cap stone is still unlocked at lev 20, but now you have an extra 2 levels to play with (get that rogue splash!).
Ever TR after that remains at lev 22 cap (with only lev 20 required for unlocking completionist?)
Other pros/cons? If TR did raise your level cap, should it keep getting higher rather than max out at 22?