If they're not gaining enough new customers to offset the losses of those that eventually leave the platform or go out of business, and if we take it as a given

If they're not gaining enough new customers to offset the losses of those that eventually leave the platform or go out of business, and if we take it as a given that over long periods of time all businesses will eventually disappear or reshuffle their technology stack, then the mainframe market is shrinking. Possibly very slowly but it's still attrition. That might make it good business for a long time but it would be a stretch to call it thriving, and it provides gentle but salient cause for businesses to consider migration.