I hope we don't reach a point where we say "now what?"
Without IEP, I can see 91s continuing to run well into the thirties. The ECML could easily end up like the WCML, with electric locos going on and on because there is nothing to replace them. Not that they need it, of course. I've occasionally considered cut-down Eurostars as a cascade (they'll be replaced one day) but they are only a few years younger than the 91s, and apparently don't fit accross the Tyne Bridge anyway, although they go to Leeds perfectly well.
There probably are options. I would go for something similar to a shrunken version of the AGV as the ultimate solution, or perhaps a bulk order of 395s for the West Riding services. If it's possible to make a tilting 395, then there is an option to accelerate services to Scotland. I would much prefer something home-grown, but I'm not sure where it will come from.