I'm not sure Apple would be wise to lock others out of ARM (if they did acquire it).
No lock-out. That part won't happen.
But Apple owning the chipmaker would give them a HUGE advantage. Apple alone would control who knows what about chip-development plans. Apple alone would have the advance/inside knowledge needed to be first to market with the best combination of SW and HW.
Competitors buy chips from Apple? No problem... just more income for Apple.
Competitors use those chips to make inferior products? No problem... those competing products will have to be offered at a lower price in order to sell and that just acts as a price-support for Apple's superior products.
Even without any lock-out, competitors will see this puts them at a disadvantage. So they will (over time) seek some alternative they can migrate to. I can see a consortium developing to promote another fancy competing chip architecture. But you know its tough to get consortium members to agree; you end up with group think and stuff designed by committees in an attempt to satisfy multiple political objectives. They might eventually succeed, but only after years of sales lost to Apple.
This could help put (keep?) Apple in the forefront for years to come. It could provide a substantial lead that would give them time enough to conceive and carry-out their next big idea. Many other companies might fail to put such an acquisition to serious competitive advantage, but I believe Apple would have a well-thought-out plan that they are perfectly capable of executing well.
There may be legal challenges (primarily to prevent the lock-out thing), but I'd bet that Apple will anticipate any such challenges and be well-prepared to neutralize them. Apple doesn't act in an impulsive, willy-nilly fashion.