Double deckers on the X4 is never going to happen, there's too many difficulties. Re-route it through Brotton and the locals will complain they can't get to Saltburn/Redcar. The 5 hasn't the time to be re-routed via Laburnum Road either.
As soon as there's any diversion in place you may have height problems. You limit what other routes the buses can be used on.
Deckers would keep worse time than what's used now, and of course there's the cost because they'd need to invest in a number of double deckers.

The best option is decent spec new buses, but we all know Arriva have this annoying habit of not regularly upgrading their fleet, and instead prefer to invest big style only when they absolutely have to, and until then flog their vehicles to death for years without so much as one new bus to be seen.