No it hasn't. "Optional" has been the status quo, meaning a lot of shit remained broken. "Mandatory" means everyone now has to deal with all the broken shit, and the stuff that re-breaks, and the stuff that gets exploited, on and on ad infinitum. This is guaranteed to make less reliable services.
Nothing gets fixed until people realize it's broken. And yet, it's also true that using TLS for mail submission and access has been best common practice for several years, with most providers specifying TLS in their connection instructions and some even deprecating cleartext. This RFC might result in more service calls as users start being told their connections aren't secure, but it also tells providers what to do to fix the problem.