Sure, it will reduce some types of soup abuse and the cookie cutter top lists will change a bit. What I mean is that it won’t remove the type of soup people usually complain about (CP batteries) while completely neutering moderate soup lists that contain multiple factions cooperating. Top lists will still not see much variation (though lists might be different from now), while the variation at the middle tables, where people like to experiment, will decrease.
I guess the main point I’m asking myself is ‘what’s the goal of this change’? It just feels the result is neither fully here (I.e. soup is encouraged) nor there (I.e. mono faction is encouraged). To me it looks like “soup is encouraged, but only as main faction + CP battery”. If that’s the goal, then the proposed rules works fine, but I’m not sure if that’s an equilibrium worth striving for.
Note though that I think it it would be a great change to split stratagems into ‘general faction stratagems’ and ‘faction command stratagems’, I.e. make certain, but not all, stratagems of every faction only usable if the warlord is from that faction.
Since some units are simply balanced around the availability of certain stratagems while others are not, banning all stratagems from secondary factions makes some kinds of soup bad while not bothering others, introducing a lot more balance problems. Other stratagems (especially most subfaction stratagems) on the other hand would make a lot of sense to only be available if your warlord is from that (sub-)faction. Though I fear such a distinction would have to be introduced by GW
and cannot easily be done as a tournament rule.