If someone could parry 4 frame pokes on reaction, they would be unstoppable. Nobody can, but fact remains that good mixups and throws isn’t a specifically P-groove problem.
A-Sakura/Bison/Blanka is a damn good team, but even a good SBB could get torn up by a good P-groove set , locked down by a good N-groove, etc. The only groove that seriously lacks the options to be on top is S, and even then people can make it work.
The most important element is the player in control.
The answer to every tier thread: Its the player, stupid.