You could indeed make that argument - but it is rather simplistic.
Demand in the home counties would be significantly harder to meet - for example there are not tens of thousands of perfectly serviceable housing units just lying around.
Additionally the land is already far more developed - just look at aerial photography of the area between Birmingham and London for example.
Then compare it to the area around Cornwall and Devon.
They are unlikely to drag shopping home on the train - also whhy would they have their after work drink in London when in an hour's time they could be having the drink next to the sea in a beachfront cafe - normally in a place with more pleasant weather?
Then why it is the poorest county in England and one of the most deprived areas of the country?
We don't live in a command economy and thus can't force industry to locate to Cornwall and noone wants to be five hours from London when you can obtain cheap land far closer (in travel time terms) in the North East.
Its extractive industries have been obliterated by the collapsing demand for British China Clay and the constantly reducing fishing quotas.
It has even been eclipsed as the source of tin (that new mine is in Devon and not Cornwall).
While it remains so isolated all you are going to get are holiday homes - that is an undeniable fact.
You can either remain in an impoverished county where the young have no prospects whatsoever or you can accept the life of the home counties which I can guarantee will overall be far less bleak.
Cornwall already has some of the highest living costs outside of London (some reports put it as the most expensive after London), and wages/average income is still very poor (again, it has been reported that Cornwall is the poorest region in Europe). The county cannot afford any more rises in living costs, most families are already living beyond their means. Jobs are hard to come by down here for unskilled/semi-skilled roles, and whilst vacancies are easy to fill, wages won't rise. I repeat: Cornwall does not need an influx of commuters pushing up prices.
Cornwall DOES need more indigenous industry. Tax breaks, relocation/start up grants etc could all be used to persuade companies to invest in Cornwall. That way, more money would stay in the local economy.
Let's look at it this way:
Family A exists in your planned future. 2.4 kids, nice coastal property, well paid jobs in the smoke. Where do they spend their money?
- House: payments go to a lender. Money doesn't stay in Cornwall
- Electricity/Gas/Water: Money goes to energy firms, some owned by other countries. Doesn't stay in Cornwall.
- Education: Chances are, children would be in paid-for education, rather than state. Some very good public schools in Cornwall, but by no means certain the kids would be sent there. Money could go either way.
- Consumables: food, petrol, etc - all goes to places like Sainsburys, Tescos etc. Supports a few local jobs, but money ultimately leaves Cornwall.
- Disposable Income: May get spent in beachside cafés, but may just as easily be spent in other attractions anywhere along or near the route of the cheap, quick rail link. Money probably wouldn't stay in Cornwall.
Family B lives in Cornwall as it should (not necessarily will) develop, with companies coming in and providing semiskilled jobs/training:
- House: Same as family A - money goes back to a lender.
- Elec/Gas/Water: Same as family A.
- Education: Far more likely to be in local education because of parents being semi-skilled workers. Local education supports local jobs.
- Consumables: Same as family A.
- Disposable Income: Far more likely to be spent in Cornwall.
Your High Speed link will therefore do almost nothing to change where people spend their money, and do so at great cost. Enticing businesses to move closer to Cornwall would be far cheaper and a better method for stimulating the local economy.
Also, travel costs will not be 'essentially zero'. Look at every major infrastructure project of the last 40-50 years and there has been a premium paid by users (Tamar Road Bridge, Severn Crossing, M6 Toll, etc). What are HS1 & the Channel Tunnel if not a high-speed transit system similar to what you propose? I don't hear of any cheap commuter tickets for sale from Paris to London.
As for the 'simplistic' argument about living in the Home Counties, there aren't 'tens of thousands of perfectly serviceable housing units' here either. Cornwall has a chronic housing shortage.
In short, Cornwall is Cornwall. It is not the Home Counties. There is no local will to change things (which, believe me, as a non-local beggars belief at times), and your plan would cause huge amounts of grief within the indigenous population. There is already a minority of lunatics who would prefer Cornwall to have full Independence, with all the issues that would cause.
Get the Cornish economy supported by industry again, and things will improve. A HS rail link to use Cornwall as a commuter belt suburb is daft.