Well, that’s what happened with me Peter - B1-B2 through input only. Of course, I make a heap of mistakes and there are things which I should be able to produce but can’t, and things which I shouldn’t but do. haha
To be honest, I think that a very high B2 or early C1 might be possible too. I’m certainly not finished with my Dutch input, so I can only imagine what level I will get to eventually and at which level I would have spoken at, if I’d held off until then.