It's a bit more complex. The licensing restriction is really just a restriction that data you mix with OSM data also needs to be ODbL, because other data under ODbL can't be incorporated back into OSM. Mixing two geodata sets is a pretty common thing to do, but you can't do that as easily with OSM data because of ODbL (I think it has something to do with the crediting).

Some companies have been interested in using and contributing back to OSM data, but didn't go further, because of the license.

Why do you think a fork that lacks the advantage of a free license can become a threat to the continued existence of a project?