I see no issue with it. The same way I see no issue selling old DOS games packaged with DOSBox. Neither ScummVM nor DOSBox are games themselves. In this case it's the content that matters.

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.

unfortunately it's not that simple, especially as long as the original behavior is still in there

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

Scummvm could adapt OpenTTD for their own working in the exact same way as OpenTTD. They did that with Ultima.