It's more that all USEFUL tech is found. It's certainly not true that only the top tiers have been explored.
Sure, if some glitch exists in a new version that makes Dan able to kill you in one touch from anywhere then that's a thing, but outside that type of esoteric junk it's all done.
There is certainly tech that's under-used, like with Doom vs flying Sentinel if you do air super and he attempts to fly above it you can DHC to your next character and the camera will drag Sentinel down into the beams during the superflash without him being able to block, but in most cases, like I said, lack of options compared to the god-tier is what keeps most characters from mattering. All of the god-tier characters except Cable can do overhead/low/left/right mixups, all of them can do 100% damage, and they either have mobility options that let them run circles around everyone else or in Cable's case anti-mobility options that mean a lot of characters just can't do anything.
I mean, you can hope for whatever you want, but as someone who has played the game since release, the character imbalance is gigantic.
That isn't to say you can't WIN with non god-tier teams, but you have to significantly outplay your opponent in order to do so. In which case, if you played a better team, you'd do even better.