The answer isn't an absolutely definitive one. Yes, we need it in the opinions of some people. No we don't, in others. Passenger numbers will go down but then they'll go up again, and HS2 is all about capacity. My own view is that we need it.
Railway infrastucture has a similarity to cars and houses, in that when stuff gets old and less efficient, yes, you can keep patching it up, but if it isn't that sound to begin with, you're better off scrapping it or knocking it down and starting again.
As regards the notion that business can work very well without face-to-face meetings, I don't agree. It CAN work without them, but it works much better with them. When you're on a 7am flight on a Tuesday morning out of LHR up to Glasgow, or Budapest, or wherever, look around. These people aren't doing this for fun. It's work or business of one kind or another. I'd rather not have to get up at 4.30am to be on that plane, but I have a client who's worth the effort (and they all are of course) then I have deemed it necessary to be there.
And as regards the cost, 100 billion is just over two-thirds of the annual NHS budget, but seven times the annual foreign aid budget. Or it's about seven weeks' worth of taxation income to the Treasury. You can make figures mean whatever you want them to mean, of course.