* It will slowly go stale, for example, it may not get ported to newer, increasingly expected desktop APIs. * It will lose users to competing software (such as your proprietary fork) which are better maintained.
As a result, it loses its relevance and utility over time. People that never update their systems can continue using it as they always have, assuming no online-only restrictions or time-limited licenses. But to new use cases and new users, the open software is now less desirable and the proprietary fork accumulates ever more power to screw over people with anti-consumer moves. Regulators ignore the open variant due to its niche marketshare, increasing the likelihood of things going south.
Harm can be done to people who don't have alternatives. In order to have alternatives, you need either a functioning free market or a working, relevant, sufficiently usable product that can be forked if worse comes to worst. Free software can of course help in establishing a free market, it isn't one or the other.
If a proprietary product takes over from one controlled by the community, much of the time it's not a problem. It can be replaced or done without.
If a proprietary platform takes over from one controlled by the community, something that determines not only how you go about your business but what other people expect from you, everyone gets harmed. The problem with a lot of proprietary software is that every company and their dog wants their product to become a platform and reshape the market to discourage alternatives.
MIT by itself does no harm. If it works like LLVM and everyone contributes because it makes more sense than developing a closed-off platform, then great! If it helps to bootstrap a proprietary market leader while the originally useful open original shrivels away into irrelevance, not as great.
We shouldn't be depending on trickledown anything. It's nice to see Valve contributing back, but we all need to remember that they can totally evaporate/vanish behind proprietary licensing at any time.