I would venture to guess (actually having read some ARM license agreements) that existing products would be fairly insulated. If Apple did this, Intel would have a serious x86 competitor, or a big guy would buy a startup and supply the rest of the industry withing 1-2 product cycles. Apple competitors would not use ARM IP so production of cores with ARM IP would drop off a cliff and Apple would be looking at a large writeoff of the acquisition value.
There would probably be a secondary request in the US and probably some interest from EU regulators on the acquisition, despite the fact that you could claim ARM has healthy x86 and other competition. Regulator interest on a deal like this would probably hold up the closure by 6+ months beyond what would probably be a 4-6 month minimum. In the cell phone land, that is a lot of time for a next-gen product cycle.
Fun to dream, but really tough to execute.