Do you think the owner would have spent a load on retractioning without checking the corrosion situation? Not likely, is it?
The decision to retraction them was taken in 2013, long before the ITT was issued for the new franchise, so Stagecoach at that time may well have had very different intentions compared to the situation which later panned out. They subsequently decided they wanted to walk away, so it isn't relevant, really. SWR now run the franchise, and operate the trains, so it's up to them to decide how much longer the lifespan of the trains will be. In any case, there will still be 455s operating for a few years yet, so there's every likelihood we'll get ten years or so out of the first ones to be re-tractioned. That's fairly respectable.
A lot of money was spent on giving the Brighton Belle units a major refurb only three years before withdrawal, and the D Stock was withdrawn about a decade before the original planned date, simply because circumstances change. If better solutions come along, they're worth adopting. The 455s are going to get increasingly difficult to operate, and however well they've been looked after, they're like ovens in the summer, lack modern information systems, and lack toilets, despite being used on some fairly long runs. I think whoever ended up operating the franchise would have ended up replacing them at the earliest opportunity. Let's get over the retractioning thing, shall we? It's still given a good few years of cheaper, more reliable operation, so what's the problem?