I'd imagine that they wouldn't do it to "take a stand" so much as "avoid the risk of getting their stuff broken in the short term" in this scenario, regardless of whichever party loses the blame game. See: the recent WordPress drama, which has turned customers away from both parties involved.
On the flip side, for user impact, it will play out like this: Some bank or other important entity could possibly, for whatever reason, continue using a (presumably expired? unless DigiCert continues issuing anyways; note that most likely, they will not.) DigiCert certificate after the cut off date, which will lead to users receiving errors. Some of them will have HSTS setup, which will lead to an emergency situation where they have to issue a new certificate ASAP, as it will basically halt their business until they do. For places where there is no HSTS, users may be instructed to simply bypass the certificate warning temporarily, and support lines will be absolutely swamped until they actually fix the problem.
The WordPress situation is quite different. You don't have to use WordPress. Users don't even know what the Web PKI is to find an alternative to it, not that there is one or will be one.
They don't have free rein to do whatever they want. If they chose to remove them, they are going to have to be ready to defend themselves in court.
If the browsers removed DigiCert, DigiCert could certainly sue based on the harm to their business. They could argue that the browser vendors were inconsistent with the application of their rules. They could argue that the browser vendors were unfairly competing by kicking them out.
Not saying they would win all these, but they would certainly fight it - the alternative would be the end of DigiCert, they aren't going to go down without a fight in whatever venue they can find.
... and DigiCert did not try to appeal the TRO.
...unless they notice the breakage, and you tell them to do so to stop it.
That all assumes DigiCert continues issuing certificates they know won't be trusted. I assume most likely what will actually happen is they have to start selling certificates from another CA, but failing that they're probably just going to stop issuing certificates before the cutoff.
I'm not arguing that it's good that a relatively small number of entities (mainly Google, Microsoft and Mozilla) decide which CAs are trustworthy, but that's all the more reason that it's important for all of this Web PKI work to happen completely in the open, so that the few who can spare the time and effort to scrutinize what is going on can help the rest of us have a safer Internet. We don't have a better solution. That's also why DigiCert issuing legal threats because they don't like how one of these issue reports makes them look is a serious problem that can't be tolerated.