Look at Germany's spending, and then predict how it will be financed a year or two into a recession, especially with falling exports. Since it's hard to cut the expenses, you either increase taxes, or you increase debt. Taxes are already pretty high and unpopular. The "balanced" budget has been a thing since 2012, but hasn't in the past 50 years before that. It's not like it's virtually unknown for Germany to take on debt.
As for the SPD being left-wing: traditional political categories quickly break down when applied to Germany, the continuum is "far left, nothing, nothing, the super majority in the center, nothing, nothing, far right". There's very little actual difference between the "conservative" CDU, "social democrat" SPD, the socially progressive Green party and the "free markets will take care of that, whatever it is" FDP when it comes down to policies, which is also why Germany's policies don't change a lot when the government changes (the conservatives removed the military draft, the social democrats cut back the social system, and the pacifist Greens & social democrats went to war in Kosovo).
> A rational policy in this environment would be to take advantage of negative interest rates to finance stimulus.
In a time of growth, record tax income, strong-ish inflation (thank god for cellphones and smart tvs or it would shoot up like crazy)?