Okay, first, Shinigami don't just achieve Bankai when they need it. It's a process that takes either extremely long and difficult or short and dangerous training. Unlike Dragon Ball, Bleach characters can't just pull a transformation out of their ass if they get emotional enough. That said, Ichigo has already achieved Bankai, and there's nothing to suggest a Shinigami can somehow lose the ability to use it anymore after they initially achieved it. So we'll probably see the real Tensa Zangetsu in the nearby future.