The issue with MIA is that it's low yielding as a stand alone destination. Had the flights been able to tie in with cruises and such, then the flight may have stood a better chance. Miami isn't exactly the top of people's city break destination, and TCX has no feed to tap into either. There are better beaches elsewhere in Florida, such as Tampa hence the rumours.
I'd dare say Fort Lauderdale could have even stood a better chance than Miami. It has more cruises should TCX tap into it, still has access to Miami, easier to get to he Everglades and is a large JetBlue hub to which TCX could feed.