The problem, as I see it, isn't that the breakup was ineffective, but that later Congresses decided to render it ineffective. They decided to make the market less competitive (or at least to allow for noncompetitive behavior) at a time when more competitive action was actually needed. Nominally, the system was opened up a bit. But as we've seen with the demise of so many dsl providers (for instance), it wasn't really very open. These crazy mergers are making it less open.
I think a MS breakup would be a good thing. I'm not sure that there' a solid legal case for it; in fact, I think the legal case is probably quite bogus. But I think that from a consumer's point of view it would be a good thing. If that doesn't happen, the next best thing would be to require that all file formats be nonproprietary and publicly odcumented. That goes for everyone, not just MS. In fact, that would probably be even more beneficial to consumers than a breakup, since it would force actual competitive behavior.