Factset's main defensive position it that they have direct feeds from most of the money managers and can do attribution analysis on their trade data overnight. That might not be as hard as getting the market data Bloomberg has, but still a pretty big step.

Also the performance attribution that FactSet provides requires constituent data from the major indexes. Money managers are particular as hell about their indexes, so you can't just provide generic indexes, unless you first convince them that the generic indexes have value. That could take years.