It doesn't look as if today's medium-speed trains to Leeds are seeing many London businesses despatching their people to create wealth in Yorkshire - there are clearly lots more Yorkshire folk going down to create wealth in London. What is the logic for assuming a complete change in the balance of the extra traffic that is attracted when the journey is 40 minutes shorter?
The general rule on the Forum is that introducing a line from a village/ small town (or improving aforementioned service) into the nearest Big City would be A Good Thing.
So, to use fictional BBC settlements, it'd be good for little Ambridge if they reopened the train line into big Holby, since people in Ambridge would have access to the jobs in Holby, people already living/working in Holby would be attracted to moving to Ambridge and commuting.
Similarly, speeding up the service from Dibley into Holby would be welcomed since faster trains encourage people (and it's an outrage that the train from Dibley into Holby takes longer than it did in steam days).
Nobody has argued that the Ebbw Vale - Cardiff line was bad for Ebbw Vale - I don't think anyone would argue that a Portishead - Bristol service would be bad for Portishead.
BUT, when London is involved, the opposite seems true. People don't want better services from Holby to London since that'll suck all of the life out of Holby, it'll turn into a "dormitory town" etc.
So goes the logic that I've seen some people use. It'd be great for Colne passengers to have a better railway to Leeds but it'd be terrible for Leeds passengers to have a better railway to London. A normal train station linking two conurbations will invigorate the smaller place but an HS2 station linking two conurbations will be bad for the smaller place?
My own personal take on this is partly based on personal anecdote. When I graduated I couldn't get a good enough job in the (provincial) city that I studied in, so I started commuting to a larger one around an hour away. That meant spending enough additional time and money each week to justify the higher salary. Cities like Sheffield attract thousands of talented teenagers to study but don't always have decent enough jobs to encourage them to remain there once they graduate (especially if they graduate thousands of pounds in debt and need to start earning proper money). It's a vicious circle - the brain drain - the talented young people heading south once they have their degrees. Employers want to attract the best so they base themselves where the best have moved to... and people move there because that's where the employers are.
At the moment I know a handful of people who travel from Sheffield to London regularly but the time (over two hours each day) means it's not feasible to do so daily. A hotel room in the capital can generally be cheaper than travelling back to Sheffield to kip at home before setting out stupidly early in the morning again.
With HS2, London comes closer to the "one hour" mark for people in Manchester/ Leeds/ Sheffield (it'd have been faster from Meadowhall, but let's not re-open that can of worms). And with four hundred metre long trains, we'll have flexibility to offer much cheaper fares too. So, commuting to London every day becomes feasible. Yes, you've got to get into central Manchester/ Leeds/ Sheffield in the morning before getting your train to the capital, but the same is true of places like Brighton/ Swindon which see large numbers commuting each day into London (and Manchester/ Leeds/ Sheffield have a lot of modern inner-city flats around the city centres, in reasonable walking distance of the station).
At the moment, a Student who graduates in Manchester/ Leeds/ Sheffield and moves to London does all of their earning and spending in London. There's no benefit to Manchester/ Leeds/ Sheffield. BUT, if they can commute from oop-north to Euston in an hour then they might remain living up here, HS2 fares being cheaper than paying London rents. So, you'll have a talented graduate pool remaining in northern England, doing their shopping here, paying their taxes here. And, once you have that hub, it'll be much more attractive for employers to locate in Manchester/ Leeds/ Sheffield - there'll be a larger number of talented people living around here - you don't need to base your business in London, you can tap into the hub of quality people living in Manchester/ Leeds/ Sheffield.
That's just my personal opinion. At the moment, someone with a First Class degree from Manchester/ Leeds/ Sheffield Universities who wants a good job will move to London and then be of no economic benefit to the city that they studied in. If we can make a journey from the M62-belt into London take about as long as sitting on the Underground from Zone Six into the City then we'll keep more of them. If thousands of people commute from Swindon/ Brighton etc into London each day then making Manchester/ Leeds/ Sheffield - London journeys a similar time/cost will be of similar benefit.
Put it this way - if faster/longer trains from Manchester/ Leeds/ Sheffield are A Bad Thing then why aren't people from around the UK demanding slower services to London? London is happening, whether we like it or not - the only realistic options are watching our best people head south, never to return, or to give them a way of getting the best of both worlds - a well paid job in London *and* a proper pint of ale in their local pub.