Only problem with 2H stab is that it was overnerfed, new animation + severe pierce damage nerf. The effect would be it will soon be completely gone, as there is currently no reason to chose a stab over a swing attack. It was indeed stupidly overpowered before, but I don't think making it completely useless is the best option. People will simply adjust and use high cut damage weapons instead, the Claymore for example has a high potential of becoming the new German Greatsword and the side-swing spam will replace the stab spam.
I've seen people using thrust effectively since the patch, its just more of a backstab/unaware opponent type move.
Spinthrust is not limited to 2-hand only, any weapon with thrust can do spinthrust. Ever seen a good pikeman fight up close with an opponent? Guess what he does? Yeah... spinthrust.
And no, it doesn't make other 2-handers more viable, what's the reason to pick a 2-handed weapon when polearm can do that and everything else far more better?
Sure, its possible with other weapons. But at least one category of weapons its more difficult now. And it wasn't just spinthrusting that was a problem, it was also the damage and effectiveness of a 2handed thrust over any other type of attack - which is why most 2handers fights were stab spamming fights. And it was stupidly effective against cavalry, too.
As for polearms compared to 2handers, I'm not sure. Polearms have always had a multi-purpose ability that 2handers lack (cav, anti cav, shieldbreakers, range, good animations for blocking - though this might be a subjective thing, polearm stun etc).
Personally I only ever chose 2handers over polearms because of personal taste - I prefer how they look and handle. In my experience post patch my bastard sword is still easily capable of fighting against polearm users, so I don't really see a problem with the categories being imbalanced.
If you think polearms are better now, just make a polearm character instead.