I support the idea of using more diversionary routes, even when it adds potentially even 2 hours onto the ordinary journey time. Many people would prefer to stay on a direct train. It also doesn’t help that the experience on a replacement bus is usually very poor, coordination is often lacking and too often not enough buses are provided to shift the number of people travelling.
I don’t however get the obsession with using the Settle & Carlisle as a diversionary route. Whilst it’s useful for freight, in the era of 125mph multiple units ultimately a direct Carlisle - London service diverted south of Carlisle would get to London quicker going via the Tyne Valley and then the ECML to Kings Cross than any diversionary route involving the Settle & Carlisle (be that via Leeds or Preston) would get it there.
A clear path across the Tyne Valley can reach Newcastle within 1 hour 15 minutes. A non stop clear path on the ECML from Newcastle to London is 2 hours 40 minutes. Add in a 15 minute dwell at Newcastle waiting for a path, and that would be a 4 hour 10 minute journey from Carlisle to London - if two trains were to leave Carlisle parallel, one diverting via the Tyne Valley and ECML, the other via the S&C to Preston, at the time the train via the Tyne Valley reached London, the one that went via the S&C would only just be passing Stafford. Of course the main issue that would prevent such use of the Tyne Valley is route knowledge - it would be unreasonable to expect WCML drivers to learn around 330 miles of new route on top of what they already sign, so for that reason alone the Tyne Valley will likely never be a viable diversionary route for the WCML.
When the WCML is shut the ECML is open and as many passengers as possible should be directed to the other route for through journeys. Using the Settle & Carlisle line is total nonsense.
Other diversionary routes are not such a detour such as those used for ECML closures and well worth using to maintain through journeys.
There have been occasions where both the ECML and WCML have had sections closed at the same time. A few years back the Caledonian Sleeper ended up running via the Durham Coast line, thanks to ECML engineering work coinciding with a prolonged closure of the WCML north of Carlisle, if I recall correctly this was when there were issues with Lamington Viaduct. It certainly would be helpful if more coordination was made to ensure that closures did not take place on both routes at the same time unless there was really no alternative - if that means postponing non-urgent engineering work on one route as a result of an unplanned but prolonged (more than 24 hours) closure on the other route then so be it.
Even some of the more usual diversionary routws are become harder to timetable with the gradual increase in the existing services along them.
This year's Cambridge diversions saw some southbound ECML services time for up to 2hr20m to do Peterborough-Cambridge-Kings Cross along with 20-25 minute waits at Peterborough.
Onlty a few years ago they were pretty much timed for around 2 hours.
The recent diversion via Leeds at weekends for LNER has seen 30 minute waits at York to match up the paths north and south of York.
On the other hand the Carlisle diversions for LNER have generally got quicker in the last few years, taking a slightly slower path straight behind a Northern service across the Tyne Valley but then slotting directly into a path north from Carlisle without any prolonged wait. Previously these diversions regularly waited in Carlisle for 30 minutes plus (albeit slightly offset by a quicker path across the Tyne Valley ahead of a Northern service) - even on the trips that were Mark 4 sets dragged across the Tyne Valley there was still plenty of time where the set was doing nothing but waiting for a path, all work to attach/detach the 67 being completed long before the scheduled departure from Carlisle.