I can't imagine the cost & objections to a southbound chord out of Exeter Central

the WoE route needs more double track as it is, without adding demand beyond Exeter. I refuse to get drawn into another discussion about Okehampton, but that budget wuold also buy a fair bit of realignment on the existing route too. Add cash for Exeter resignalling into *any* increase in traffic, no matter the direction.
Err, I said redouble the WofE line (along with Okehampton) to run additional services, the only reason I mentioned the southbound chord (and that would follow at a later date) from the WofE line was as an option to increase the number of London/Plymouth services using the otherwise poorer value for money DAL.
Poorer value than the Okehampton route (even then the Okehampton route study didn't look at what impact running through services from the WofE line and certainly not services from London would have on the value).
As had already been established the impact of other schemes would be limited, even the DAL would only gain (at best) 6 minutes and had some cost estimates of up to £3.1bn in the 2014 study. (Via Okehampton in comparison was up to £1.2bn).
I'm happy to discuss other options where it would allow extra services to connect to Plymouth from London where the journey times aren't too dissimilar to the current services.
When is been discussed before I am accused of only suggesting via Okehampton as it happened in the past. I then explain that there's no spare capacity out of Paddington and there's no proposal to provide any (unlike from Waterloo where's the potential for 8tph in a post Crossrail 2 world, and some key locations - like Portsmouth 2tph and Guildford with 4tph - aren't likely to be able to get their average of +1tph for each 2tph they currently see - or at least pre COVID), likewise there's no other London station worth capacity nor the ability to run services of a comparable journey time.
Even if there was scope to turn services at OOC, the capacity around Reading West would likely stop additional services running.
As such whilst the WofE line was used historically, the reason for suggesting it now is because of current constraints.
The DAL would only ever see it's business case improve if there's more services using it. To justify that you need to build the numbers of passengers traveling between Exeter and Plymouth; the best way to do that is to improve the frequency of services with a journey time of circa 1 hour - which even with the DAL would likely run into difficulties between the DAL and Plymouth.
Lengthening the Castles (or more likely their replacements) and the Voyagers would help, however even if you had 3tph with a total of 1,800 seats vs the current 3tph with a total of 1,050 then rail growth at 3% per year would make trains just as crowded (at least seated capacity) in 18 years with those longer trains as is the case now with the existing rolling stock.
Given that Crossrail 2 is (at best estimates) also 18 years out we're taking long game here (as I intimated in my post). Probably a late 2040's early 2050's opening date.