Backdoors only really became a pressing concern due to ubiquitous Internet access. When I first setup a Windows VM and install whatever application software and updates, its Internet access is through a public VPN only. And it contains no information to tie it back to me.

Before I put any sensitive information on it, I kill its Internet access and never reenable it. So there is no way to exfiltrate data that I care about. Any produced information leaves via a local Samba share.

Leaking fixed identifying information about my hardware, or forming a side channel to a new VM instance would violate this security. I doubt the TPM would store persistent personal application data, but I don't need to be the first one to find out.