I am playing a game with 4 stars.  My goal was to play strictly diplomatically.  But I wonder if there is a point of diminishing returns?  You see, I am allied with half of the other players.  The "other" four are at war with me but they are also dead.  I have 20 diplomatic points with 2 races and nearly max pacts with them.  I am very powerful.  So I am basically watching my allies duke it out while I do things like

1) pay tribute to keep relations high as I don't do missions (because I can't attack allies)

2) really micro manage my envoy ships as they may soon be on systems that become neutral

3) look for available systems to take over (see #2 above)

as everyone is my ally, there is at least one bad thing I can think of: when you take over a planet that has tactical and logistical elements in place, they must stay there.  they cannot be deconstructed or destroyed. 

Playing the diplomatic route means burning a lot of logistic slots for envoys.  that means less for a battle ship.  If an ally goes to war with me all of the sudden, I would have to scrap my envoys quickly and buff up my fleet.  I find that at this point, my fleet doesn't fly around via way points or anything like that.  It sits clumped in one system.  There's nothing to fight.  It's a wait and see game.

I am playing this game, but I am wondering, what's the point of playing this long?  I think when the AI beats everyone else but me, it will start a war.  If I didn't pick up planets that turned neutral via attack from others, then those others would occupy them and in the end they would be 2 to 3 times as big as I was.