You already get an advantage for being older team. You know the ropes and prolly have built some seats to arena. We don't need ingame cash bonus to keep managers.
I agree that, perhaps giving out supporter points (for example one per day), would earn you 3 months in 3,3 years.
It's summer, people are vacationing and not really interested in sitting behind the computerscreen. They will need something to play again, when autumn comes.