I’m not sure how to interpret this other than Atari not wanting to compete with OpenTTD on Steam.
A lot of the fan-driven reimplementations of classic games are trivially derived works, because people seem to think that the copyright only covers the pixels in the originals and if you replace them you're fine.
On game engines, reimplementations are not derivations at all but tools for interoperability, totally legal to create. From Wine to most of the stuff of https://osgameclones.com, to GNUStep against NeXT/OpenStep API (and Cocoa from early OSX) and so on.
If you could sell Cedega back in the day you can totally sell OpenTTD with free assets, period.
The entire PC industry exists today because of cheap IBM BIOS clones from Taiwan.
What I'm contending is if the assets are actually free. And just because they were all created by volunteers and contain no data from the originals doesn't mean that they are actually free. The rules around derived works are complicated, and too close homages have been found to be derived works, even if there is no actual copying.
If this were to go into court, things that would matter would include both "how visually similar do they look" (the answer is "very"), and "was the artist aware of and did they refer to the originals while doing their work", (given it was done by volunteers who are enthusiasts of the original game, the answers are almost certainly "yes" and "they can't prove they didn't").
And on those facts, the new art is a derived work of the original and falls under its copyright.
Artistically speaking FreeDoom it's closer to Half Life and the like than Doom but here's the catch: playing Strain for instance won't look like a mess, but different, a bit like a demaked Half Life (or a game from its era with the Unreal engine) but not a copy.
I sincerely doubt that. Unlike the FreeDoom assets, it is too visually similar to the originals, and visual cohesiveness with existing materials (which were created to fit the style of the originals) is a point in favor of it being a derived work, not against.
Forgot to reply to this part: And the reason those clones exist is because multiple companies reimplemented the BIOS in a clean way, where they had one team produce a clean spec of all the interfaces, and then sequestered a different team, who could attest in court that they had never worked with, seen or in other way come in contact with materials related to the IBM PC, to produce a replacement BIOS based on only the given spec. The clone makers that didn't go to all this effort were sued out of existence.
Do you believe that the free assets produced for games generally meet this standard?
I’m usually sentimentally open to IP rights being overly constrictive in the current regime, but faced with a company that owns TTD™ saying “hey, instead of going full lawyer nastygram to avoid confusion, let’s work this out so people get your stuff when they download ours”…seems pretty nice. Like I can’t imagine Microsoft allowing alt-universe OpenWindows™ on the Windows Store.
I personally think it is the copyright that is the most uncertain. Firstly, there are probably quite a few venues around the world where Atari might be able to take this up, and quite a diversity of precedent between them. Historically Atari litigated in the US - in 1981 they lost https://en.wikipedia.org/wiki/Atari_v._Amusement_World on a case someone infringed by copying their look and feel without copying any assets. Other precedents in that jurisdiction have found it's not infringing if similarities are inherent to the subject matter of the game: https://en.wikipedia.org/wiki/Data_East_USA,_Inc._v._Epyx,_I.... but similarity of art style is copyrightable: https://en.wikipedia.org/wiki/Tetris_Holding,_LLC_v._Xio_Int....