While I see your point, if you look at eve, there have usually been two or three mega-coalitions that control everything and trying to make your way into nullsec you have to either bend over or somehow gain a strong enough following to fight the big guys off, and in this game that would probably be even harder, especially if you give more rewards to someone who already controls a quarter or half the entire server will make that faction a juggernaut that can wipe people away with hardly a though.
Now i know most people just think some enemy will already be there or always be there to challenge an empire that gets too big, or it will have internal struggles, which is going to be true some of the time but you cant count on that all the time and you definitely don't want to feed the faction who controls everything already so those who do rise up get crushed.
I do support the idea of having Guild>Kingdom>Empire though, albeit the game isnt ready for it, wait for more guilds to be part of kingdoms than guilds not part of kingdoms then have something like that, and while i still dont support the idea of giving an empire more direct benefits make it potentially easier for players to interact, like special buildings for trade between kingdoms and guilds or something, it would be good to have more community brainstorming on the idea.