I think your main question is what happens when the MMA agent is depreciated later this year, correct?
The 'Learn More" link in the portal will explain this in greater detail. MDC is moving to an agentless model that uses a combination of agentless scans and collaboration with MDE features.
While Agent onboarding is enabled, and Azure policy will redeploy the extension every 12-24 hours.
Defender for Severs P1 and P2 will continue to use the MDE extension to enable MDE.