There have been things like this i know, but it's different, and i think it would work.
First of all, im not sure if there are ads on supermechs, if there are, there aren't many.
Anyway some more advertisement wouldnt be a bad idea, such as little popups every once in a while you can close. I'm sure they'd be kinda annoying but they'd make money wouldn't they?
Anyways to my real point: Every either 100,000 or 200,000 exp you get a random mythical for free. This makes long time players get better stuff, and the long time players see more ads, therefore making money.
It will also encourage players to play more, so you can gain mythicals after campaign, instead of only having to pay.
Not sure if this is a good idea at all!