You'd still keep the UPS. Industrial sized UPS's already contain large batteries capable of powering significant loads for extended periods, but their key purpose is to keep the power going when the incoming supply has issues and to deal with all the switching and synchronisation needed. For a use like the ROC, think a large room full of batteries and switch gear, not a little black box at the bottom of a pile of computers.
If you were going to implement a large battery storage system it would likely replace the generator in this sort of system, rather than the UPS. A separate component designed to provide longer term power. The big issue though is charging - once the batteries in this sort of device have run out, that's it - you're out of power - and it's going to take a while to recharge for it's next use too. Generators for backup power are designed to be refuellable while running - so can provide power indefinitely.
But then your battery becomes the single point of failure instead. Designing resilient power systems is hard work, and relies as much on operating process as it does system design - you've basically got to design a system where every single component can be swapped out while the system stays operating - and you've got to do it in a way that is safe both for people and the equipment it's powering.
