In theory a separate company who's core competency is running IT systems should be less likely to mess this up, not more so.

However, retail banks are rapidly becoming sort of API hosting companies for the processing of financial transactions. Their core competency IS running IT systems.

If none of RBS's physical branches opened tomorrow, the UK would probably tick along just fine, with some inconvenience caused to people who deal mostly in cash. But fail to process payments for 24 hours, and the whole UK has problems. If this had gone any longer than it did, it's not hard to envisage a run on the bank and a domino effect to other banks. It's really quite scary how badly we depend on these IT systems, waaay more than Google or AWS.