In reality the thief’s role has never been to +1 fights, but to +1 ONLY 1v1 fights, because when a thief turns a 2v2 into a 3v2, it’s way too risky to engage, because the CC, extra sustain the 2 give each other and the collateral AoE damage on a node is at best prolonging the fight to a point where the thief is running out of fuel and at worst too much for the thief to endure.
In light of this I always wonder why people keep saying 1v1 ability is not important, when in fact if there were no 1v1s, thief would have no impact on matches, as there would never be an opportunity to 2v1.
That alone should be enough to understand that thief needs the ability to 1v1 just like any other meta build.