But we come back to same argument getting any money will be difficult therefore the idea of money for multiple options is just shear fantasy, that's why it has to be the right solution for the long term.
Depends on how you view long term, are we thinking the next 20 to 40 years, or are we thinking the next 50 to 80 years?
Right, so lets say the DAL gets built (say with linespeeds of 90-100mph) in a hurry to just about cope with what is needed and then in 50 years time there is a proposal for a new HS line running to Exeter is proposed, then chances are the upgrades will stop at Exeter and the trains will then trundle onto Plymouth and Cornwall.
Conversely if the mainline is still routed through Dawlish, then there comes a big advantage in continuing the major upgrades to Plymouth (especially if there are closures of the line most winters by then) with linespeeds of over 140mph being a possibility and depending on how fragile the line through Dawlish is it could justify the new upgrades to occur sooner than they otherwise would have been. However in the meantime Plymouth has never been cut off from the network.
The big risk with the DAL is that will be it in terms of upgrades (other than electrification, maybe a bit of redoubling and a slow increase in the number of services) for services west of Exeter for a long time. Conversely the Okehampton reopening means that further upgrades will be required in the next 30-70 years, however by then there could be a lot more passengers (35 years of 2% growth doubles the numbers of passengers).
With double the passengers linespeed improvements become a lot higher value in terms of benefits. Therefore a HS tunnel between Exeter and Newton Abbot built in the next 50 to 60 years with a journey time of about 6 minutes, compared with the best current time of 18 minutes then becomes more likely. That would clearly be better than the DAL being built in the next 10 years with a journey time of about 15 minutes.