Tilt would be great in the Highlands - lines where faster services would be really useful, but where normal speed improvement works could not be justified. Doing the (pretty minor) tilt adaptations as part of the electrification work would be very smart for the lines north of the Tay - which are currently scheduled for electrification in the 2030s. That would then be a sensible use for some shortened 390s.
More generally, mostly, the top-end main-line trains get moved to secondary routes after about half of their working life. Look at all the Mark 3s that aren't on WCML any more. The Mark 4s and Class 91s aren't going to be scrapped when IEP comes in, but they won't run on ECML either (GEML? MML? who knows?)
So why would it be a surprise that the IEPs/AT300s and the 390s go and head for secondary routes when HS2 arrives?
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Will this pay off £80 billion?
No, but it will bring in the maximum revenue that half-empty trains running in off-peak hours can bring in. No-one is expecting to pay off the capital cost of HS2 in fares anyway - if it gets paid off directly at all, it will be through the increased tax revenue from the increases in land values and the increases in economic activity.
Unlike some here, I expect peak-time short-notice fares (today's Anytime Single and Anytime Return) to go up for HS2. If you want to be in London for 9am, you can either get a 7:40 on HS2 from Manchester, or a 6:40 on WCML. That 7:40 will be able to charge a premium over the £332 return that Virgin currently charges.
But the rest of the fare structure is set because people won't get off-peak trains unless they're cheap - so any HS2 operator will have to sell those tickets at the same prices as current off-peak advances.