I think this whole discussion is too hypothetical until we see the next support introduced into the game. They could even decide to rework all the current supports. The only recent supports are Kharazhim and LT and only one of those is a healbot. I don't think you can even say Bliz were aiming for being able to play Tyrande solo support with the levelling changes. She clearly is OP right now with the ability to out heal the healbots while still having one of the best stuns and traits for bursting.
Tass can still put up good damage and has excellent utility. But I would not consider him a solo support.
You might not - but he is being used as a solo support at the pro level quite a bit now.