It's a fair point to make: mandatory copyright transfer in combination with copyleft means the copyright holder can at any moment decide to change the license. Not retroactively of course, but you'd be left with the choice to fork, to buy a license, or abandon the product. None of those are great options for anyone serious about making a contribution.
Copyleft can work great but it works better if the copyright stays with the original authors. This makes it impractical to re-license because you need all copyright holders to agree to that, which in the case of most OSS projects is virtually impossible. Imagine tracking down everyone who ever contributed to Linux. Not going to happen. And as Oracle has learned the hard way, forking is what happens if you mess up. Copyleft serves users of the software.
IMHO the long term success of OSS projects requires a diverse community of contributors. If a project has value, people and companies will step up to keep it going. Companies don't last forever and their fortunes or priorities might change. Having copyright transfers basically means one company calls all the shots and if it suffers, their software and its wider community suffers. This dis-incentivizes externals to contribute; weakens the community; and can become a problem for projects being long term sustainable or indeed profitable as most of the burden of maintenance is for the sole copyright holder.
Of course it's not too late for MongoDB to swallow their pride and fix this PR disaster. Lets be frank, all this negative news around their product has got to be unpleasant for them and their shareholders. At least, I doubt that they expected or wanted this to happen and I think they should be bracing for more bad news when e.g. some of their big accounts start eyeing competing products because of this mess. The fix is obvious: move the other way and go with a business and customer friendly license (Apache 2.0 seems to be popular for this) combined with closed source components/add ons for commercial customers.