Cooperation in itself is an accomplishment; this is why you get a greater sense of accomplishment by teaming up to beat something (that necessitates it).

Although I don't think solo play should be innately discouraged (many people simply like being in an active, changing world, they don't necessarily like working with all the other people in it), I don't think saying "go solo it" is an acceptable solution to grouping problems. There should be plenty of things that can be done solo and plenty of things that can be done in a standard group. They need to cut back on stuff that is difficult without an alliance, as alliances tend to be more frustrating as you're depending on a larger number of people to not mess up, and that's assuming you can gather that many in the first place.