Probably not very many.
All the examples they posted already have free servers and it's not like allowing people to continue using them is going to result in the original companies publishing the full code. Rather they will have to continue development to match the last patch like they have been. What will change is the argument over intellectual property.
Back in the good ole days you worked on those projects and released it for free, then people started coming together in communities that posted advertisements, then people starting forcing "donations" like PlanetMinecraft's nested ad-links, and now it's Patron's pay-me-to-provide-updates-or-I-won't. With the original corporations unable to issue take down notices to freeservers charging monthly connection fees you'll see an almost immediate capitalization of mostly stolen content. I'm not say everyone will quit, but someone else making money off your content you worked on is a great way to kill your motivation to work on anything related to it. This will inevitably turn to the worst sort of thing as each "free" server will turn to monthly fees to pay developers but each one hordes it's development instead of sharing even through both are probably doing nothing more than playing catch up to someone else's IP that was released in one of the official patches.
The DMCA exemption needs to include a very nice non-profit clause to it. Force them to work for free and they will work out of the love for the game. Sharing becomes encouraged.