????? You are mistaken. MS varies enormously in how fast and how severely its symptoms develop. In some patients it is utterly catastrophic. In others it can be manageable. Generally worsens over time. I have had both a friend and a relative die of horrible cases of MS (well complicating conditions usually arise toward the end). Both ended up completely immobilized. Yes, it can be catastrophic and fatal, in some cases.
I should have said it’s not an immediate death sentence. The symptoms and complications can vary quite a bit from person to person. Quality of life can suffer a lot, obviously.