Good thing for their label to lock them down, basically ensuring they'll have the full length career of a normal successful boy band.
But the 36 month thing could cause problems. I could see Zayn wanting to walk away to pursue his own career but being forced to stay because the others want their money. Zayn has a breakdown and quits anyway, and the other resent him even as they put out new albums because they aren't as successful and they lost out on their additional money.
Ten years later, Zayn after a successful solo career that has now faded wants to get together for a reunion tour, and although the other three have moved it past it and are willing, Niall is still bitter over what happened

and the tour never happens