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.
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.
I assume they will take the original and most likely unchanged TTD binaries and package them together with DOSBox and that's it. It's something that one dev could do in a weekend.
This doesn't feel right for me. OpenTTD is so much superior in every way compared to the original TTD, that noone in their right mind would ever play the original. So Atari now, while spending zero effort compared to the years of work that OpenTTD devs put in, will basically sell OpenTTD as if was their own creation. People who buy the new TTD will simply play OpenTTD anyway, since it's so much better.
I might be wrong, but it feels like Atari are like parasites in this situation feeding off the hard work of OpenTTD devs.
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.