Think I know what I'd rather have....
Modern banking platform, running a micro-service architecture on a private cloud on commodity hardware, likely in others' high availability data centre in an active-active configuration, and employing industry standard middle-ware that its easy to hire staff to operate OR
The traditional mode o banking IT of shelling out every 10 years for the next generation of IBM mainframe because it can run running creaking code, quite possibly dating from the 60's, and in COBOL; staffing this operation is a significant risk in its own right and change takes literally years (Ask RBS about this).
I reckon TSB is in a much better place long-term than the other major banking groups who are working out who how on earth they're getting off their legacy systems. The appetite for a radical re-platforming is much reduced.
Much talk of building their own challenger banks from scratch (or most likely, with more expensive IBM middleware) and migrating customers slowly. Several of these start-up incubators are in the Old Street area in London - strangely enough just down the road from Monzo :)
I understand that when Lloyds and TSB merged, they merged onto TSB's IT platform. Which Lloyds is now left with ;)