It’ll be years before he’s not good enough for the top 30. Dave Gower is 35 next year and he could probably get another NRL contract somewhere for 2020 if he wanted it.
I think there is a little difference between the expectations of pay for Mannah and Gower.
Mannah has been a Top 17 player for 10 years, including captaining his club. There is an expectation of a completely different minimum level of pay for a player with that experience.
Gower has never been paid an amount that would've had him in the Top 20 players at any club he has been at for his entire career. Realistically, the way that the incomes have gone up at the lower end of the squad, he's probably still getting a pay rise every year.
I can't see Mannah sticking around in the NRL on money that isn't considered Top 17 money. I suggest he won't be a a Top 17 player at any club in the NRL in 5 years time (which would make him the same age as Gower is now)
I suggest Mannah's future is some decent pay in the UK.