Computer based systems will go down. anyone that tells you differerently is lying. How fast they come back depends on how much you have invested in paralellity, it's testing and/or availability of critical human support and spares.
They who demand max downtime guarantees in the 100'th of a percent are just asking to be lead up the garden path. Every airline should therefore have a manual minimum service alternative, or max one that is only dependent on local computing power. That be standalone tablet, pc or mobile. Sample; With passenger lists for the day stored locally at start of each day, only last minute changers would be denied boarding, reducing customers displaced. And this goes for flight ops as well as ground ops, now when so much has to communicated electronically same day to agencies like Eurocontrol.
BA (and Amadeus) seems to not have been willing to make a clean break from it's "mainframe" based systems maybe because that would have lead to guaranteed downtime. This has lead to a patchwork that is not set to diminish over time. Sometimes its better to make a clean break with the past. And even Amadeus now have alternatives. The problem is finding somebody with the stomach and power to schedule, and willingnes to take the flak for, the temporary disturbances it will guaranteed lead to. And when the worst happens, at some stage it will take to long to recover the mess of today. So better to reposition, clean out and create a fresh start for tomorrow. Sad for todays customers, but at least tomorrows won't be affected.