With Reinhardt, couldn't a team poke at his shield till it was broken or near broken while theirs was intact before engaging/pusbing with speed boost.
That's exactly how it works. The Rein needs to be able to balance shield health in a way so that he can protect his team and not lose shield when they can be jumped on. The ideal way things should work is that Rein's shield dying is an advantage a team can work with. Right now Rein's shield dying is kind of a death sentence.