MIT/BSD licenses allow for proprietary forks. The most popular BSD-based kernel is Apple's, which isn't distributed under the BSD license. The most popular GPL kernel is Linux, which still is.

So with the BSD license somebody can take your work, improve it, distribute it under a proprietary license and supplant you in the market. Compare macOS or iOS installed base to FreeBSD.

But then the codebase is henceforth tied to that company. If Apple is dead in twenty years then probably nobody is still using the iOS kernel (and not because they switched back to FreeBSD), but if Google is dead in twenty years most likely Linux is still popular all over.