Having a livable life is worth it to them, because earlier drugs did not work.
Tysabri is almost never a first-line treatment, and often is used for
patients whose disease is at the secondary progressive phase. No other treatments have been identified for this group, except alternative, generally stronger, infusion drugs that in their milder way act much as chemotherapy does. Yes, risk is involved.
As for Tecfidera, I feel you are taking reported data out of context.