Azure Managed Instance Zone Redundancy
The ability to make Azure Managed Instances be zone redundant to match the HA capabilities of SQL Database seems logical.
So I actually hit up the PG on this as I ran into this myself.. The reason this isn't an option is because they are using the Azure Basic Load-Balancer internal to this tool.. It's the same reason you can't setup Geo-Replication over peers (you have to use vNet to vNet VPN). They told me the reason was cost - but honestly the limitations make SQL MI less appealing as a solution. I opened this feedback asking them to switch out the ALB.. https://feedback.azure.com/forums/915676-sql-managed-instance/suggestions/38175127-move-to-standard-loadbalancer-internally