The movies may well introduce more villains, or have Megatron playing a minor role (even if it is a flashback, just to have him on screen)... but the point I was trying to make was from Hasbro's perspective - no matter what Bay does, they will want to sell as many toys as possible, and that means Optimus, Bumblebee and Megatron.

Look at TF2 & TF3 - as people have pointed out above, Megatron was a sidekick to The Fallen and Sentinel Prime.... but who had more toys and merchandise in the stores?
The Fallen had...
a Voyager toy (in three colour versions),
a Legends toy,
a Robot Replica,
and a Robot Heroes figurine. That's it.

What did Megatron have for TF2?
A Leader mould (done twice),
a new Voyager mould,
a redeco of the 2007 Voyager mould,
a Legion mould (done at least twice between US & JP),
a FAB mould,
a Powerbot,
an Activator,
a Robot Replica,
three different Robot Heroes figurines,
a Battle Charger,
a Speedstars Stealth Force vehicle,
and an RPMs/Speedstars Mini-vehicle (done four times).

The ratio is closer for TF3 between the Megatron and Sentinel Prime toys... but was still in favour of Megatron (7 to 6 - there were over 150 TF3 toys but only 13 were of the two main villains), making Megatron the bigger "villain" cash-cow for Hasbro.

That was my point - ignoring how well someone else can be written in as a character, or how much Bay wants someone new (since he keeps trying to get rid of Megatron), Hasbro will make sure that Megatron is on screen in some way (flashback or in the story), so that they can sell more Decepticon toys, to the kids and parents that Hasbro "feeds off".

The proof is the excessive number of Bumblebee and Optimus toys... produced for the majority of consumers who aren't fans (returning consumers).