In their current state, I don't think earn ops are a good thing. They promote awful gameplay and usually gives inexperienced and bad players high tier tanks that are difficult to use. Instead of limited time earn ops, why not make them last forever but make them take alot longer to complete and have more difficult challenges as you progress, for example:
1: Destroy 50 enemy vehicles
2: Deal 50,00 damage
3: Earn 10 High Caliber medals
4: Earn 10 Top Gun Medals
(The requirements will obviously be alot harder this id just an idea of how they can progress)
You could also make them more relevant to the tank you're earning, so if you're earning a Light Tank you'll have to use a light tank for the op, I feel like this will give more incentive to learn how to play a certain class better.
You should also tier lock them, so if a player hasn't researched a tier 8 TT tank they can't earn a tier tank. Putting inexperienced players in high matches doesn't help anyone.