Yep, that's what I'm trying to figure out atm. It keeps the modified one as a .old and downloads the default one again 🤔
Well I found a way around that way back when Big Picture first appeared and I tried to mod it but it's not ideal.
You can set TrustedInstaller as the owner of the file and then revoke write permissions so Steam doesn't even have the right to rename it and it will eventually launch. Although that triggers the "Updating Steam..." dialog every start and it will stay for 30-60 seconds as Steam tries to get the original file back and fails.