The Brummbär is supposed to counter infantry and weapon crews. By the time it arrives even an opponent that has been outplayed should have access to Cromwells, T-34s, Shermans at the very least.
The unit arrives when there are mutliple options to counter it.
If you can't stand the Brummbär standing up to unsupported AT guns I assume you are for an ISU-152 and Croc nerf aswell.
Nerfing the range and survivability is just another nail in the coffin that is Ost lategame. But every crutch unit that somewhat kept Ost in the game is nerfed.
Croc already got nerfed into the ground. ISU might warrant a nerf in team games considering how strong it is on certain maps.