That's exactly how Survivor is played. How many alliances have lasted from the first show to the final 3? I can't remember many, Yul & Becky maybe but that's about it.
Alliances change after every TC, that's what makes it challenging. This is especially true after the merge because people start thinking about who they want to take with them to the final 3.
I would agree that Tai not taking Scot & Kyle with him to the final 3 was dumb, but maybe Tai got a gut feeling that the gruesome twosome wouldn't want to take him to the final 3 because he hasn't really made any enemies? Then, seeing Julia cozy up to the boys probably got him thinking even more. The final straw was Scot being bossy and not inclusive with Tai.
If Tai would've went along with the GT's plan, the final vote would've been:
Scot: 3
Aubry: 3
Tai: 2
That would've resulted in a tie. He's in a power position again to play the SI or not. Alternatively, he could let the tie play out. Ironically, he would be holding the power position in a tie as well, because this is how I think the vote in a tie would've gone:
Cydney > Scot
Jason > Aubry
Joe > Scot
Julia > Aubry
Michele > Scot
Tai > ??? but probably Scot
He could vote Scot out or force another tie in the revote. There's no way he'd force a tie in the revote because then all 6 of them minus Kyle would draw rocks.
However it's played out, the writing's the on wall for Scot. At least the way Tai played it, he'll be able use his play to garner favor with the other alliance.