Current system:
doing lower level missions than your current level grants lesser rewards
pros:
encourages players to play higher level missions
cons:
creates an endless cycle of rendering old content useless
Creates an unnecessary demand for new missions since the choices to earn money will be very limited to the newest high level ones, and players will get bored much more quickly.
punishes players for leveling
Optimal system:
Mission rewards do not reduce with level
unlock additional missions varying in difficulty as you level up, with similar reward amounts.
hard missions award more gralats, but this does not mean people wont run easier ones because they are less likely to fail. There will be hard missions at both high and low levels
Pros:
All missions will be worth running, over time creating a myriad of missions to play and choose from.
Players can form large groups to tackle very difficult missions for excellent rewards, or run easier ones for easy gralats
Creates an incentive to level up to unlock new content
Cons:
New players can't participate in high level missions and squads might add level requirements to join.
If the exp system becomes too grindy then players might have to repeatably play the same missions to unlock new ones. This should be avoided by making leveling accomplished at a reasonable pace.
tl;dr
I'd much rather be able to play 30 different missions as I please than the 3 newest ones and see 27 old missions all gone to waste because they offer no incentive to play.