Full support from Proxmox isn't cheap, compared to even the new prices on VMware, if you look at the per processor cost that small businesses often have.
As I said - it depends on processor count. I know a number of small businesses that will be paying $5k/year for VMware, not much more than Proxmox top tier (which is what they would want).
Proxmox is about $1500 per processor, so would be $3k-$6k/year for these businesses. That's a trivial difference when you look at VMware already being installed and running, no transition costs, no risk of migration. You'd burn up a few $k difference with a single issue.
Then there's backup - I doubt VEEM supports VM's in Proxmox yet, and how do services like iLand work with Proxmox? These are associated costs, with migration costs and especially, risks.
Frankly, as much as VMware annoys the shit out of me, I couldn't recommend migrating to Proxmox for those businesses, today. At best I'd recommend planning a transition when they need to upgrade servers, and do it early as a parallel install to give transition time for the business.
SMB doesn't have the luxury of test labs for this stuff - they don't have the cash flow/finance room to justify it.