The license agreements and ToS for all of this has various "we can terminate this in any way we want at any time we want for any reason we want" clauses, which is effectively a magical bulletproof solution in many countries.
in this case using the term "buy" and in their ToS use the terms license / termination, might find the front and center "buy" term causes the license / termination terms to be nulled out, as the concept of "buy" doesn't fit with the concept of "license" and "termination".
If those companies exist, is there any practical way for customers to know which ones they are? If not, customers are not free to refuse to engage in practice.
The only freedom that customers (=citizens) have is to engage into a cartel (=state) to impose their collective interests (=democratic choice) on companies by force (=law).
Companies are then free to refuse to engage with states which have such customer-protective clauses in their laws.
Yes, and you can found more of them, too.
> If those companies exist, is there any practical way for customers to know which ones they are?
Yes.
GoG (good old games) sells you games without DRM. There are others like that, too. Some companies, like id, open source many of their works.
You can buy DRM free music and videos.
You can still buy movies on DVDs, which still can't be disabled remotely. DVDs are very digital.
Yes, you can complain that there's no 'significant' movie marketplace without DRM. But you have to complain to and about your fellow consumers: their patronage ultimately decides which suppliers become 'significant'.