So let's stick to the facts.
Megatron - DEAD. Shockwave - DEAD. Starscream - DEAD. Soundwave - DEAD.
Ironhide - DEAD. Sentinel - DEAD.
Michael Bay said in an interview, that what dies in DOTM, stays dead. They will not magically be brought back to life. Let's say that will stay that way in the future.
What is a Transformers movie if there are no Decepticons?
For most people, Megatron, Shockwave, Starscream and Soundwave are the Decepticons. Period. Then there are others of course, but how many seconds of screen time have they had in the trilogy all together?
If this one makes more money than ROTF (cause that's what Paramount will be looking at) and gets to the Top 10 list at the All-Time World Wide Box Office list, then Paramount will wanna continue with the franchise.
Bay has said no more. Shia has said no more. Let's say they don't change their mind.
With the ending DOTM has, Paramount has a choice to make if DOTM is successful around the world.
Try to make a fourth one with the remaining characters or reboot the whole thing.
What do you people wanna do?
1. No reboot. Period. Let's leave it at this.
2. No reboot. Let's continue the story with the remaining characters and a new director.
3. Reboot with a new director and let's start all over again from the beginning.