Agree with a lot of the above. I used to work for German Rail (DB) and got free passes to travel through Europe - so I did. I frequently used to travel back to Ireland by train & boat (Oostende - Ramsgate, Holyhead to Dublin - even the boats were free) and I was amazed at the state of the railways in the south east back then (1994) - no true intercity service, old grubby slam-door stock, trains seemed to stop everywhere. On that first trip I got into Victoria, then got the Vic line to Euston, then on to Holyhead on a really nice HST (class 43's, or Intercity's) which were good trains. Then I moved to England in 1999 and and have lived in the S.E. since then.
Her Majesty's Railway Inspectorate (now the Office of the Rail Regulator) basically gave the mandate to replace the old slam door trains (which were only really left in the south east) for safety reasons - not just the lethal doors, but also the couplings, which were not able to prevent the chassis of one carriage slicing into the next one in the event of an accident.
This was supposed to have happened by 2002, but didn't as the new trains were still being built (ordered late as Connex never wanted to spend a penny (literally) on the south east. The first new trains that arrived were put on the north Kent route and the Chichester/Bognor/Brighton lines. The rest of Kent got the hand-me-down Networker trains, the class 365/465/466 horrible commuter things in late 2004.
The High Speed line was driven by the Government's aspiration to connect to Europe's train network & make it appealing. The route chosen was predominantly a political one, as St.Pancras is geographically illogical for France! Passing through Stratford - now there's a way to boost the area.
If you live in Manchester (for example) then you're right, this probably won't make much sense. However if you're in the South East & at the mercy of Heathrow, Gatwick, Stanstead, Luton or Docklands, this is a viable & welcome solution for travelling to the continent.
I was at St.Pancras for some "Volume Testing" last Thursday (i.e. they invite you along, you check in & go through security and then travel on the train to Stratford & back. We did this three times (unpaid for the day) which basically amounted to a grand-scale simulation. St.Pancras is impressive, there's no denying that - even if it did cause the architect to go a bit mental! The Eurostar trains are the original ones & are starting to look a bit tatty now (plenty of corrosion when you look closely) but hey, they're better than taking a slam door to Ramsgate.
I assume people saw the TV programme - a lot of aggression there, particularly by the project managers (some of whom I know) but there's no denying the end result.
Whether or not it could (or should) have been done cheaper, I'm not sure. It's a lot of tunnels, a lot of bridges, and a lot of buildings. Personally I think it's a feat of engineering that will remain in place & be of great use for generations to come. London is still on the map, despite Ken Livingstone's best efforts to close it.