I can’t find a way to edit my original post (or add tags) so I’d better say that Cristoffs’s diary and the Imagico blog (especially the triangle) as linked above are key reading. While there is the alternative approach of trying to solve problems by governance as was done with iD, it doesn’t raise the profile of alternatives, which is less serious for iD as other editors are well enough known. A building block for something like this could be infrastructural support like tile servers or vector tiles.