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.

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....

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.

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