That might be surprising for developers that released a Unity game back in, say, 2015, when Unity CEO John Riccitiello was publicly touting Unity's "no royalties, no fucking around" subscription plans. Now, even developers who paid $1,500 for a "perpetual license" to Unity back then could theoretically be subject to additional per-install fees starting next year (provided their game is still generating sufficient revenue and installs).
This reminds me of a story from earlier this year from Wizards of the Coast, publishers of Dungeons & Dragons (and subsidiary of Hasbro). It hinged on exactly the same semantics.
The short version is that, in 2000, Wizards of the Coast released D&D under the "Open Gaming License (OGL)," which gave third parties explicit approval to make and sell their own material using most of the D&D content, under a perpetual license. Cut forward 23 years, and lots of major publishers got their start making D&D supplements, and continue to use the OGL because (a) it's a cover-your-ass license in case they tread into a legal gray area, and (b) allows them to open up their own content to third parties. Plans for an update OGL leaked, with predictably dogshit terms that I won't get into right now, but essentially killed the license as anything anyone would want to use. The malicious part was that they would be "de-authorizing" the OGL 1.0a, because while it was a perpetual license, that didn't make it irrevocable.
(IIRC, it's also a legal argument based on case law established after the OGL was written. Not a lawyer, though.)
Predictably, there was a huge backlash. WotC backtracked, and even gave up ground by releasing a bunch of stuff under the Creative Commons. However, the OGL is still dead, because third parties can no longer trust that WotC (or Hasbro) won't try this ratfuckery again. (Sound familiar?) Lots of products were subtly rewritten to no longer need the OGL, and several publishers worked on an industry license amusingly called the Open RPG Creative License, or ORC.
The thing is, D&D's going to survive this a lot better than Unity. The business model was to sell D&D and D&D supplements, they only indirectly benefited from third-party material, and people are still going to make D&D stuff because it's D&D. Unity's entire business model relies on licensing, so if people stop using it... that's it.
