This one time - in band camp Optimus died. I know this because it happened near band camp.

 

Megatron, Starscream, Soundwave, Shockwave, Barricade, Sentinel and Cybertron died. No one to give orders and nothing to fight for. Time to go home. Wait, WE DON'T HAVE ONE!!!

 

They'd be plenty to fight for even though all is lost. The Autobots and humans don't believe in imprisonment, rehabilitation, redemption, or anything of the sort (even despite evidence to the contrary), so the Decepticons would have to fight just to survive. Survivors could seek revenge and attempt to destroy the Earth/civilization purely in a "if I can't have nice things no one can" manner.

There are Decpeticons scattered all throughout the globe. I suppose it could be handled like how the opening of RotF implied, but it's still an incredibly clumsy way to end the (so-called) "trilogy."

 

It's not just about when the main villain died. It's about the Decepticons' objectives and whether or not they could still succeed. Let's analyze each film, in fact...

TF1: The 'Cons wanted to take the Allspark, which was the only reason they were fighting the 'Bots anyway. It got blown up WITH Megatron. Fight over.

ROTF: The 'Cons wanted to activate the Star Harvester. Again, only thing they were fighting for. It got shot up by Prime within 10 seconds of the Fallen dying. Fight over.

DOTM: The 'Cons wanted to use the Space Bridge. It got shot up by Ratchet and Bee within 10 seconds of Sentinel dying. Fight over.

 

Ye this makes sense ^^
But cos there "objectives" were destroyed doesn't mean that they would jst stop mid-fight and walk away . They would most probably keep fighting for a while until they decide to give up ( they're enemys , they're going to keep fighting even if it's for nothing I would think )  

 

It's not that the main villian is dead the battle ends. From what we got in the movies, many, if not all of the Decepticon forces in the battle zone are dead before the fight with the main villian happens...

In the 1st, the killing starts before Megatron arrives, and in the end he is fighting alone.

In the 2nd, some decepticons are killed during the Egypt battle, but all are finished off with the bombing... only then the Fallen appears and along with Megatron, fight Optimus.

In the 3rd, some protoforms and their gunships are taken down during the initial Autobot/NEST incursion, as the fight progresses the Driller,Starscream, Soundwave and some more protoforms are killed, followed by Barricade and his subordinates. Then Optimus arrives, takes down another bunch of decepticons and Shockwave, before fighting Sentinel... at the same time the Wreckers battle with the Clover-Bot. Also there are Decepticon Gunships that are called by Sentinel for support, but are taken down by the Tomahawk missiles already en route.