Riker's island is very poorly designed. A big part of the problem is that it is quite inaccessible from the rest of the city. This has a ton of knock-on effects:

1. There's a severe shortage of guards. Anything that could make working in the jails more attractive will help, and a 2-hour commute is not attractive.

2. Getting medical service, prisoners with court dates, lawyers, etc. on and off the island is really really slow.

3. The jail has insourced a lot of services (for example they run a large bakery). These get staffed by corrections officers. This both makes staffing harder, and the existence of these plum positions has weird effects on the politics of the corrections union (people with power in the union are not actually working as guards and are out of touch with the reality of the jail)

Moving to a model like the Tombs, with the jail directly above the courthouse, makes way more sense.

In addition, the buildings on the island need a serious revamp (e.g. there is no automatic access control, so every major door needs a guard physically standing there to let people through and climate control is broken). As long as you need to seriously renovate anyway, might as well just build a new jail where it should have been put in the first place.

Source: I read a lot of blogposts.