None of the limitations of open-source licenses apply to the authors themselves. (author = the person or organization whose name appears in the copyright notice). I.e., you can have a MIT/GPL/AGPL licensed project, but have a "premium" fork/derivation/later-version of it that's completely closed source.

I actually see this as a valuable incentive to open-sourcing under MIT -- if a commercial provider of your software emerges, it will help you test/prove that a commercial market for your software exists, after which point you can completely close-source it and pivot to purely commercial competition.

Open-sourcing, then, is basically baiting the waters to see if anyone sees commercial potential in your work. And the minute that's validated, you get funding and start your company.