Cabin heater failure causes the entire drive system to fail. This is Christmas light string level engineering. You are double fucked if your heating fails in a blizzard.
But you're right - it's terrible engineering to have the entire car shut down just because the in car heat fails.
This kind of kludgey, fiddly, brittle, and complex engineering is quite common in German, Italian, French, and British cars.
Especially modern German cars. Brilliant to drive, laughable to maintain