But Windows 7 was just Windows Vista with some relatively minor changes and it was fine.

The problem with the 737 MAX is that Boeing tried to make a plane with a completely different airframe and added a bunch of software to try to make it fly exactly like second generation 737s so that pilots didn't need to retrain. But the abstraction was leaky and it turns out that when you have a different airframe, you have to retrain your pilots.

That said, Boeing did some janky stuff to get this nonsense certified by the FAA and the whole design probably needs to be recertified, which will take years.