Obviously the types of supported virtual hardware will be different between hypervisor types, but that's usually something that can be dealt with separately.
Yes? Do that for the hundreds of VMs, with a minimal downtime, bonus points for a dumb C-level on your neck.
You would quickly learn what "just converting disk images" is not enough for moving between hypervisors.
Just as an example: last year I moved a company with two Proxmox servers to ESXi. It took a 3 months and another server as a staging point. A minuscule in modern times 1TB+ VM took a whole weekend to be moved. Of course it didn't work in that time.
Why?
And that was with the help of a backup-recovery solution which did support Proxmox, mere 6 months before that I would had stuck with an agent P2V migration of a VM to VM - which would had involved way more moving parts (can't P2V/V2V to a hypervisor which is not even installed yet) and more pain.