Megatron would have gotten rid of Starscream sooner, if he wasn't shorthanded on Earth during the first two years.

 

Yes, maybe.

I have a different pov regarding why Megatron keeps Starscream around that most might disagree with: I think SS is the best replacement as leader if Megatron should fall in combat (pre movie). It's his only option for the Decepticon cause to continue. We've seen what the lack of leadership can do to the Decepticons on Charr (post movie). In the Sunbow G1 universe, Soundwave and Shockwave were great lieutenants but not inspiring, Megatron knows this. It's like a father training his idiot son to hopefully take over the family business, he'll suffer through a lot of mistakes but it's the only way to keep it in the family . To his credit, Starscream is the most ambitious and scheming of all the Decepticons. Like the Sith Lords of Star Wars Universe and true in the animal kingdom; if you are able to overthrow the alpha, you must do so for the health of the clan. Wolves all work together for the pack but the Beta wolf will someday challenge and maybe even kill the Alpha wolf but the Beta is allowed to try. If the Beta fails, that wolf is still a part of the pack and remains so until the next time. All to make sure the pack has the strongest leader possible. Nothing personal, it's rather altruistic. Megatron is just insuring the survival of the Decepticon cause having his "replacement " near.

 

Pretty sure Cyclonus or Scourge would've killed him before Galvatron did. Id bet they'd be less tolerant for Starscreams b.s.