MIT doesn’t govern at all how modifications should be shared, so some third party/company is free to just take your code and relicense it to sell it, with or without their modifications applied. That’s just a consequence of the the license:
to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software
Or consider cases where you’ve made some nifty new algorithmic discovery - MIT doesn’t prevent a third party from swooping in and claiming a patent on that discovery, collecting royalties in the process (of course, the patent office in theory checks for prior work, but in practice…). See also the commentary of the FSF on it (in spite of their questionable stance on copyleft):
For substantial programs it is better to use the Apache 2.0 license since it blocks patent treachery.
No, under these considerations, I cannot with good conscience say that the MIT license guarantees that what used to be freely accessible, also stays freely accessible.