Debt snowballs over time, its easier to create more debt when you're in debt because in order to borrow money you have to pay higher rates of interest + you're paying interest on the debt you already owe.
You're making the argument about the conservatives having doubled the debt in 9 years yet you're clearly support JC, a man who's pledging hundreds of billions of additional spending. Realistically we'd be talking about 2-3x the amount of borrowing that's currently happening, if not much more.
And maybe you're right in that i was being a little disingenuous as increase in social service spending generally hasn't out matched inflation, but to act like the Conservatives are completely gutting/purposefully under spending (as many labour figures suggest) is also disingenuous.