I use GPL precisely so that others cannot appropriate the work without contributing back. If I was okay with the outcome you describe, why not just use MIT in the first place?
With the MIT licence they can distribute proprietary software based on your work and not have the source available, as this is not required with the MIT licence. So you have no way to get the changes or improvements other parties have made if they choose not to give them to you.
Take what I am saying with a pinch of salt, others can give a better explanation. I'd suggest looking it up yourself.