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.