I wasn't trying to be rude, just explaining the reasoning from an economic perspective. Economics is pretty brutal - living in a world dictated purely by financial consideration would be a hellhole so we have to try to reach a balance, like not killing people off once they're no longer economically beneficial... An economically ideal human citizen would arrive in the country with an extensive education we haven't had to pay for. They would work in a revenue generating job for many decades in perfect health. They would then drop dead the day after they retire. They would save little, hate to travel and spend freely within this country. They would have some extremely healthy, intelligent children who they would pay to privately educate and ensure were hardworking and law-abiding. Any of us who don't fit this pattern are a bit of a nuisance really!
If you're not contributing a lot more in tax then when you worked, you're probably not as 'beneficial'; when you were working, you weren't just paying tax you were also providing a service, which is more what keeps the economy ticking than tax. I don't know offhand the current income required to become a 'net contributor' but last I looked it was 40-something-thousand. Let's says £45k, though it may be higher by now. This is the yearly figure if you're in work - so if you're 35 and start earning £45k, you'll be a net-contributor for this year, but you'll still be massively in deficit for all the 34 years you've taken more than you've given. Not many people get to the point where they ever pay back all that they cost before they got to the net-contributor level, but if they do, they still need to maintain that level forever. In order to retire without taking more than you've given, you'd have to have not only paid back all the money you've ever taken (equivalent to the tax on <£45k a year for their life so far, as things used to be cheaper) but also pay ahead for all that you will ever use, however old you get (equivalent to the tax on >£45k a year as prices will increase). At this point you'll basically break even. You'll obviously need to pay a great deal more if the country's going to get much financial benefit from your taxes. And if you modelled the impact of the necessary additional housing, infrastructure, resources etc for replacement workers you'd need to be contributing vastly more to make your tax more useful to the country than you working. Working doesn't just generate tax, it's the primary economic engine. Building houses won't in and of itself build any growth, it's the knock-on effect of having a large, secure, productive population that is claimed will help growth. This is very debatable though, many models show the damage to other areas like environmental will negate any positive impact.
I've not heard the £2.5m figure but it works out to slightly over £80k annually for 30 years, which would correspond roughly with the net contributor figure over a working lifetime (£45k for 50 years) - but as before, that figure would only hold true for the years you are earning it. For it to cover an entire lifetime it would have to be approximately £45,000 for 80 years, so £3.6m. That's of course an extremely rough estimate.
Again, not trying to attack you - I don't blame you at all for stopping work and enjoying your life. I don't think forcing people unhappily back into work is any sort of solution, I think we need to be trying to make it so people love to work and dread the idea of retiring! But that may be a long way off...