Legally, MangaDex does have to take stuff down when someone submits the DMCA. Doesn't matter how popular or old the series is. Also, having been here since the site started, after Batoto went the way of the dodo, I know they've always tried to respect IP holders' takedown requests. So not really sure why people are surprised when stuff gets taken down because of a DMCA.
That said, I do think it's a terribly look that many of these publishers & IP holders can apparently find the resources get stuff taken down via their legal people, but seem incapable of finding the funds, interest or both to do an official release. This is stuff that is being made for digital release, which means it's not terribly hard to make it affordable, or even cheap, and still turn a reasonable profit, even when hiring a decent team to the work. That gets cheaper if you go with AI slop, but I know I'm not the only one that will refuse to patronize someone if their product or service is supposed to be text heavy and they make zero effort at quality in that regard. Again, as I said in the last thread, I wish more Japanese publishers would learn from Shonen Jump. Yeah, they have their issues and the translation quality for some titles leaves quite a bit to be desired, but they certainly show that they understand the audience and get what they need to be a reasonable alternative to scanlation options.
Granted, I'd agree that the current regime for copyright is pretty BS. I'm very sympathetic towards authors and other creative types and agree 100% that they should be able to make a living off of their work. There are some where they only have one commercially successful work and some get the worst luck, where that work only takes off years or decades after its release and they might have bitten the dust by then. That said, the duration for copyright is excessive and 70 years after the creator's death is absolute nonsense, if generations are 20 years, that's 3.5 generations of after the author dies. Sorry, I can see giving the authors family a chance to get something out of what they've done, but it's beyond stupid when you have estates that never met the creator or their kids just being able to ride that success and societies are much poorer for it. Given, that you have a bunch of estates now that don't even understand the ideas behind the work, they are now gatekeeping for financial gain, that they never built themselves. Then it gets stupider when you add corporations to the mix, that usually have the funds to throw things out the wall until they get several commercial successes that easily erase any funds lost on series that were revenue negative and of course, you got plenty that screw the creatives over to get the most money.
Anyways, I digress, I'd encourage anyone not happy with current copyright law to consider pestering those elected to represent them in government to change things. Also, encourage them to find ways to pester copyright holders into doing official releases. Obviously, don't be a dick about it because that's going to be counterproductive, but it is an avenue, and the outcomes may end up surprising you. Yeah, most cases they probably blow you off, but you never know when they will actually act positively on what you tell them.