Well, I get net through telekom. As it's an old contract, I shouldn't be affected for some time. However if they change contracts, I'm pretty sure I have the right to switch to whomever I want.
As several isps already made a point to say "we won't cap anything" (of course, hoping to catch all those that leave telekom) things will be ok.
I don't agree with the telekom argument though; the German internet has so much free capacities at all times that any significant increase in traffic costs merely cents and telekom is already asking pretty high prices for their connections.