We don't need him. But had he stayed, imho he would've been a much better player now, because Bayern Munich can't possibly develop a Barca youth product better than Barca itself, the club that made Thiago a young star in the first place. A lesson for other La Masia players. You don't live Barca for Bayern in this era of football and end up on the winning side.
Anyway, I'm sure that leaving Barca for Bayern has hurt his development. He isn't much better now, than he was back in 2013. That says a lot about how wise was his decision. Instead, watch Roberto, and his development in the last 6 months. He already did better for Barcelona in the last months, than Thiago ever did. And if he continues like this, he will be a better player than Thiago anyway.