How do you feel about old Lucas arts adventure games that are purchasable on gog and other platforms and come bundled with scummvm?
However in OpenTTDs case, the entire implementation is original (including the new high res assets).
I would have 0 issues with this TTD/OpenTTD situation if OpenTTD was left on Steam as-is and TTD was a separate purchase that granted the original assets for use in OpenTTD.
and ... successfully defending the originality of the reimplementation as a standalone work would be a risky and costly legal endeavor
(even though the standards for originality - "is it transformative enough?" was more lenient when OpenTTD was made than it's today)
Scummvm could adapt OpenTTD for their own working in the exact same way as OpenTTD. They did that with Ultima.