I understand that people who have not upgraded risk being cut off from the web, but I can't help but wonder if this action compromises the safety of everyone el

I understand that people who have not upgraded risk being cut off from the web, but I can't help but wonder if this action compromises the safety of everyone else just to save those people. If there's a state which can forge a SHA-1 certificate for MITM, how hard is it for them to abuse this fallback feature to compromise modern clients?