I would have had IC225s loco-swaping at Edinburgh too, but they are also needed on Great Western as the only push-pull 140mph stock we have. There aren't enough 225s for both routes so if you left them on East Coast you would still need IEP for Great Western's Swansea, Cheltenham and Weston-Super-Mare routes (therefore restricting electrification because of the cost of clearing for 26m coaches (I've heard a tunnel around about Narberth will need widening if IEP is to go down there, which is going to be expensive and Paddington to Cardiff/Swansea is a long way to run a bi-mode Voyager or a diesel (Intercity 125) under the wires)). The way I suggested, you only have 125s under wires between Weston-Super-Mare and Bristol and between Newbury/Reading and Paddington. You only have bi-modes (for Costswold line) under the wires between Oxford and Paddington, if you could attach a dragging loco at Oxford I'd use EMUs instead (perhaps a follow on order of 395s, with end doors rather than 1/3 2/3 ???).
If IC125s can be life-extended to make it to 60 years old, then so can 158s. That would take them to 2049. Good thing too, in my opinion they are irreplaceable. The more recent units built to do the job of 158s (eg. 175s) are ridiculously overweight. Rural lines need something really lightweight to minimise track wear and with the best possible fuel economy to keep costs down, and being lightweight Sprinters are great. In other words, 158s are to a rural line what an Intercity 125 is for a non-electrified Intercity route, totally brilliant. Hopefully by the time they are replaced there will have been a lot of electrification and thinking will have moved on to allow a replacement to be built which isn’t any heavier than Sprinters.
Given electrics are supposed to have a longer lifespan than diesels I think we can give Intercity 225s 65 years, taking them beyond 2050. I sure hope somebody fits regenerative breaking to the 91s, that would make them very efficient indeed (as it is they are about as efficient as Pendolinos, which do have regenerative breaking).
158s aren’t suitable for rural lines, IMO: Their acceleration is too slow for regular start-stop routes, and their internal arrangement isn’t really geared towards carrying a large amount of luggage or large items such as bikes which are so much a feature of the rural routes favoured by tourists. Plus they must surely be more expensive to maintain than a basic, no frills 150, with larger engines and air con, etc. I would be quite happy to see Pacers replaced on rural services by 150s (There is a very similar total number of 150s to the total number of Pacers), supplemented by 153s and 156s, through investment in new stock or electrification on the commuter routes currently operated by 150s, but once the Sprinters of classes 150, 153 and 156 become completely life expired then new stock for the rural routes will have to be considered.
I would really hope that Scotrail have electrification to Aberdeen in their medium to long term plans. Once the HSTs are withdrawn, it would be a lot simpler if much of the need for bi-mode on the ECML was removed with the OHLE running all the way through to Aberdeen. For as long as the Sleeper trains exist, which IMO is dubious once the mark 3s are in need of withdrawal, there is a diesel loco available at Inverness for drags of the London-Inverness train north of Edinburgh, or Perth which is as far as I can see electrification ever reaching on the Highland main line. Modify a couple of 67s with Dellners, and they can make better use of themselves by hauling the Sleeper during the night and the London service during the day.
With a little bit of infill electrification, such as Leeds to Colton Junction, and transferral of the remaining stretches of diesel operation to an open access operator, such as the Lincoln service, all East Coast would need would be a squadron fleet of about forty all electric IEP trains to replace both the HSTs and 225s.
With all these proposals of life extending various train types, it’s starting to look like some of the trains of a similar vintage to me (I'm a child of the Sprinterisation era) are going to be keeping going when I’m into hip replacement and Zimmer frame territory! The prospect of stretching out the life span of top flight Intercity trains to anything up to sixty years of doing the same duties they always have done is simply ridiculous, and shows the current organisation of the rail industry for the shambles it is.