You hope something extremely disruptive occurs, probably technological, to make living and therefore a reasonable pension cheaper, or you keep raising pensionable age to stop the cost of pensions rising. Importing young people means that without the disruption you can increase the pensionable age at a slower rate, or ideally not at all while you wait for that disruption to happen.
There must be a more recent infograpohic than this one, but this is quite current and illustrates what a burden pensions are on our spending.
https://drive.google.com/file/d/0B8dY-3hzSftCMng5TFBkaFdSb3M/view
The ONS released new demographic statistics today.
http://visual.ons.gov.uk/most-affluent-man-now-outlives-the-average-woman-for-the-first-time/
In the last 15 years the average lifespan of men in the UK has gone from 72 to 79.1. Extrapolate that (assuming, linear regression) for your 40 or 50 years and most men will be living until they are 100. That may sound far fetched but start to look at the advances in medicine such as nanobots and it's much easier to be convinced. That is potentially a hugely disruptive medical technology, and it may be that a hugely disruptive economic technology doesn't appear until a few decades after that matures. We aren't dying quickly enough and that big pension circle in the welfare infographic will continue to grow. We need to raise more money through tax to pay for it. There is certainly an argument to make sure big business pays for more of that, but that seems to be a relatively unpalatable direction for any major political party.
So in the short to medium term we either we raise taxes on individuals or we get more people with the drive to create economic activity and grow revenue through taxation on that activity.
That's my simplistic view anyway.