If the enemy team focus' on it, but generally speaking the rest of your team should be giving them hell when their busy trying to whittle away the shield. The point isn't that the shield lasts forever, but without ults coming up constantly, it can take a LOOOOOOOT of damage before the team wears it down enough to move forward for the team kill, all while the enemy team is shooting at them back through a save spot.
The problem I have with Reinhardt isn't that he's overpowered in the usual sense, but without ults, his shield has been made much more effective. It's to the point where if one team is running Reinhardt while the other one isn't, the game can basically be considered over. It's like the new Speedboost, it's become the most important ability to have.
As a result of that, Reaper just has a harder time getting through, since his job is to flank. As is, it's more useful to keep distance and just stare enemies to death with your super death machine assault rifle from a distance, especially since you can self heal.