One RULE book is feasible. Unless they truly did have different systems in each of the books. The thing is that they don't. So far AoR has exactly the same rules as EotE . Some people (Myself included) think that there should be Setting specific systems that plug into Main rules to make the over all Star Wars Setting work best. Duty is a start. And they have said that more rules for Capital scale ships is coming in the beta. The only other thing I can think of that we could use would be Mass Combat rules and how PCs can interact with them.
Yep, instead of one core book plus three or more setting books, which has been done before to no greater effect, they chose to go a different route. Too bad for those that wanted that core book. Guess you can either vote with your wallet and not get it and that'll show them. Or you can get the game on the off chance you will enjoy it. Let's be honest here, even if they had done it your way, you would have bought multiple books anyways, because you love the premise of the game. But unless the game doesn't meet FFG's expectations, they're are not changing their course any time soon.