Not an expert but I am a patient.

Immune system reset is already a type of therapy for MS. Lemtrada and Mavenclad are two brand name drugs that work on this principle. You don’t take them for life. That’s good because they make you very ill, but after 2 rounds of treatment you can go years without needing any further therapy.

Could the measles virus work better than these drugs? Maybe, but all the reasons at the top of the comments present challenges.