My understanding is that Dalton Maag has a "perpetual" contract to support and extend the Ubuntu font family. Exactly what the terms of that contract are are not public, of course. But you can visit the project page and clearly see its ongoing development.
In any case, once the font is released under OFL, it doesn't matter so much if Adobe stops contributing to it, because others can. DejaVu and the Croscore fonts (which we discussed in June: https://lwn.net/Articles/502371/ ) are demonstrations of that. Which is not to say that it's a good thing when contributors (corporate or otherwise) drop out; just that "achieving their goals" is not an opportunity that vanishes if someone stops working.