The money is coming from somewhere. If it wasn't lent to the government, it would be spent in other ways. So the only reason that it should cause a recession is if the government is spending it in better ways to grow the economy than the private sector would/could (e.g. because the best investments are infrastructure related).

But if it's actually providing a boost to economic growth, it also means the debt isn't actually unsustainable. If you boost economic growth this year by 0.1%, and afterward it returns to previous growth rates, then the economy will stay 0.1% larger than it otherwise would have been perpetually. If your interest rates are comparable to the base line economic growth (true now, last decade they have been lower), and you take a twenty-year view, then taking on 5% of GDP in debt to generate a one off 1% growth boost to the economy is a no-brainer. The real issue is to understand when the government (vis a vis the private sector) actually can spend money to generate genuine extra growth, and when it cannot (e.g. if it only shifts growth that would happen anyway forward you would expect growth to dip after the initial boost, but that would be true whether the deficit spending stops or not).

The argument against debt is often not based in economic sense or sound science (remember the debacle when a widely cited study on the problems with high debt turned out to be based on an Excel mistake?). In my view, it's based on an aversion to the government having too much spending power. After all, a version of the argument, above was used by conservatives to sell tax cuts that were supposed to pay for themselves. They didn't because the tax cuts did not actually generate growth, they merely redistributed wealth to the top. For deficit spending, the evidence that it can boost economic growth is much more robust.

The real debt crisis will hit once countries start to seriously shrink...