As much as I’d love to see it go open source, my guess is it would probably require a major effort to convert such a large long-running commercial code base over to OSS. Likely they’re encumbered by sublicensed chunks of various components which would never fit GPL or similar. Large codebases are usually a messy hodgepodge…
On the PC I only ever encountered single users here and there and they were a pretty rare breed. I guess Modo might have had more of an audience on the Mac but it won't be enough to keep it around in some form. Just checked the Foundry forums first time after years and am almost surprised to see that Mari is still alive…
Modo's codebase might actually be workable since it is a "young" application compared to the usual dinosaurs. As I remember it was a 2004-era restart of Lightwave modeler with some of the original developers and supposedly with lessons learned taken into account. Question is if there'd even be anybody really interested in…