My issue with this argument is that I'm not sure how much of OpenTTD is their IP. OpenTTD has been development for so long that I doubt that any original disassembly remnants remain in the latest version of OpenTTD. The only true piece of IP that OpenTTD may use is the name (the TTD part of OpenTTD) and the graphics, the latter of which being the more important one. However, as far as I know, OpenTTD devs have created their own version of all the assets that are also much higher resolution compared to the original. As a result, I see OpenTTD as an entirely separate game, that's been heavily inspired by original, but is its own separate entity.

Also "TTD" is hardly a name you can claim. It's three letters that don't make a word.

As long as it officially stands for nothing, perhaps. See DotA vs Dota2.

KFC would like a word.

If you take an essay, and rewrite every paragraph and also add some new words, then it's still plagiarism. Perhaps not under copyright law, but ethically. OpenTTD goes beyond "heavily inspired" because it is intended to reimplement the original game.

I am sympathetic to arguments of the form "It was abandonware," "Copyright lasts for too long anyways," etc. But I don't think you can claim OpenTTD owes nothing to the creators of TTD. OpenTTD was meant to replace TTD and would not exist without it.

It's not plagiarism when you openly credit the original.

Making a clone of a video game, even with some substantial changes, may not actually be legal https://en.wikipedia.org/wiki/Tetris_Holding,_LLC_v._Xio_Int....

What "leniency"? It's not like OpenTTD contains any TTD IP

that would require some confirmation from a court

the standards for originality changed substantially twice in the timeframe we are talking about (recently, in 2023, the Supreme Court made it more strict, reversing a ruling from the 90s)

but still, what counts as sufficiently transformative is a very complicated question, especially when it comes to this kind of thematically identical but still creatively innovative derivative works

I remember how many years artists spent on the NewGRF packs, to make them high definition, what to add, what to keep, what to reimagine, and how! and of course the same goes for the game mechanics, the UI, the ... pathfinding, the economics (prices for freight, but also now there's a supply and demand system too, right?)

but the original behavior is still kept, so that's more clearly derivative (a copy), and in this case the new additions might be a new original work on their own, but still for OpenTTD to be able to be distributed without infringement of the original TTD it either needs to license that work for redistribution or remove the infringing parts.

(and of course it's a layman's analysis, so ...)

The Atari that may have paid for development no longer exists. This is a skin suit of a legal entity.