Economist here, clearing the deficit doesn't mean zero debt, it just means the debt wont get any bigger after 2020. Apart from during the two world wars our debt has usually been about 40% of GDP and the 2008 recession sent it up to about 90%.

The BoE is due to increase interest rates in the next 1-2 years to fight off inflation and this will bring about the next economic downturn, probably within 3 years. It's unlikely to be as bad as the last one which was catastrophic but we don't really know, it may be even worse.

The UK already has higher borrowing costs than the US despite having a better credit rating. The US lost their AAA rating after the last recession and it would be a disaster if the UK lost ours. If we ever defaulted on it our debt the entire economy would crumble.

Reducing the deficit is also important to maintain confidence in the pound, which attracts investment. So basically small austerity measures now can avoid a potential disaster like we had in the 1970's. What Labour is proposing is that they will let our national debt of 90% grow for 5 more years, the Conservatives say they can halt it within 2 years with slightly more austerity.

Personally I favour the Conservative budget over Labour's. The last five years have been difficult for some people but the Conservatives have unquestionably done a good job with the economy and job creation. They've cleared two thirds of the deficit, I would rather they clear the rest before the economy slows down again than trust Labour. Ultimately it will be lower class voters who will decide this election and they will be the ones most effected by austerity, so I fear they will favour Labour's short term view. Most people over 50 who remember real poverty will vote Conservative.