"You don't almost triple the national debt in a decade by 'paying interest'."
Actually, yes you do if the nominal interest is compounding faster than you can pay down the principal; it's grade school mathematics.

Mulroney inherited Revenue - program expenses of - 2.7%. It fell to -1.2% in his first budget, -0.2% in his second, and went into surplus of 0.4% in his third

Consolidated deficit went from 8.1% in Trudeau's last budget, fell to 6.7% in Mulroney's first, to 5.7% in his second, and 5.1% in his third.

So if Revenue - Program expenses is positive 0.4% and the Consolidated budget in negative 5.1%, what is pushing you to deficit? Answer: Interest on PET's debt.