How to configure encrypted SQL Server connections to prevent MITM attacks?
When I saw this question, I opened SSMS with the intent to read the help documentation on the "Trust server certificate" option only to discover that this is an undocumented feature with no description of what it does on MSDN. My guess is that it can be used to trust expired certificates or those which can't be validated by walking the chain of trust. Since there is no chain of trust for a self signed certificate, SSMS trusts it when it verifies the signature using the public key. 

I think your suggestion would make an excellent server configuration option for high security implementations of SQL Server. It seems like you want a configuration option to reject all self-signed certificates, since avoiding those is the only real way to mitigate a man in the middle attack. It is exactly the concern that you raise that causes me to recommend configuring the Certificate tab with a domain or public CA certificate.