[giaour]

If app usage is not steady 24 hours a day (e.g., the 100 concurrent users all live in the same hemisphere and don't use the apps much outside of business hours), you might see some cost savings from not running excess infrastructure. You'd have to run a cost projection to make sure; it's entirely possible the serverless option would still be more expensive (even before you factor in the $$ for the migration effort).

You can scale down app service/VM based infra either on a timer or in response to metrics, so it's not like serverless is your only option if cost is the motivator.