The question, which I've answered, is does the reopening to Tavistock make the business case to close the gap better - yes.
Does that mean that I think that there are other schemes which should go ahead before this - yes. However that's not the question being asked.
Do I think that the DAL is a good idea - yes, however probably only in a post HS Wales and West world when it would enable more express services to be run. The other way it could be useful is when passenger numbers have built up, so there's a bigger group of passengers so it improves the business case.
I can both support the reopening of the via Okehampton route (as it has a better business case than the DAL, and an on a similar level business case if you run no services over it) whilst understanding that other improvements are required as a higher priority.
I'm clear in my posts that it's most likely over a longer timeframe (I've suggested 30 years), and that it's worth looking at the business case in a post Crossrail 2 and WofE redoubling.
In such a world you could run the current Exeter service as the stopper along the line to provide interchange between places like Honiton and Tavistock to provide good local interchange. Whilst looking at would an express train from Waterloo would further add to the business case.
Currently the journey time from Waterloo is 3:22, with just redoubling that could cut it down a little, then removing stops it could get it down to around 2:40 which whilst slower than the express from Paddington (circa 2:05) it is on a par with the Exeter semi fasts (circa 2:30). That's not going to get you many from Waterloo (but you don't necessarily need that at there'll be plenty wishing to get to Basingstoke) however from Clapham Junction there could be scope to attract more people from stations which connect to it as they didn't have to get to Paddington.
Of course that is only looking at headline journey times, a lot of the fast services are going to be fairly busy already, so little scope for adding more people to the network. Therefore if you're looking to get to Plymouth and there's a slightly slower but marginally cheaper service you may well for to take it.
Now whilst they may mean that you are taking passengers off the fast services, that's kind of the point, in that they are already fairly full. By taking passengers off those services it allows other passengers to take their place.
Yes that's not worth doing if the fast services and the semi fasts average below 50% seat occupancy, but as they get close to being over 70% the risk reduces. As you get higher still it's probably a good thing to do as there'll certainly be untapped demand.
It would, because the journey times would never come near the main line.
Can you remind me what the report from 2014 said was the increase in journey time for 220's running as an express (without a reversal) compared to the mainline?
Now consider that it said that a reverse at Exeter (something which would be required if you are extending WofE line services to Plymouth) would be?
I know the answer to these questions (spoiler they basically balance out), so if you are going to run services from WofE line (which you'd only really want to do if there was at least some more double track sections so you weren't reliant on everything running perfectly to time) to Plymouth then the overall journey time would be about the same.
Even allowing for a stop at both Okehampton and Tavistock would be about an extra 5 minutes (+3 minutes for a station stop at each), so not really worth the effort of changing trains (even if there was a cross platform change available leaving 3 minutes after the scheduled arrival time - which would save 7 or 8 minutes however even then few would make such a connection integral to their journey without being confident that they'd make it or have a suitable backup).