Please note that even GNU themselves require you to do this, see e.g. GNU Emacs which requires copyright assignment to the FSF when you submit patches. So there are legitimate reasons to do this other than being able to close the source later.
So yes, I trust a non-profit, and a collective with nearly 50 years of history supporting copyleft, implicitly more than I will ever trust a company or project offering a software while requiring THEY be assigned the copyright rather than a license. Even your statement holds a difference; they require assignment to FSF, not the project or its maintainers.
That’s just listening to history, not really a gotcha to me.
Some GNU projects require this; it’s up to the individual maintainers of each specific GNU project whether to require this or not. Many don’t.