The point is that while you can spend a lot of money you can't spend infinite money. There's a finite pile of euros. Every euro you spend on something stupid is one less euro you can spend on something smart.
Take nuclear fusion as an example: Supposedly, this is a case where the projected investments necessary to go from theory to practice just haven't been made. With return of investment only anticipated in the far-off future, it was decided that the money could be spent in better ways.
Surely countries can invest more if those projects are 'more value creating than other investments' and if it's rational, but the notion that countries shouldn't be afraid of debt either is pragmatically no helpful.
All of us are just a few irresponsible financial decisions away from being Greece, Venezuela, Weimar etc.. Stable fiat currencies are a precarious thing, which why we are so cold and 'small c' conservative about it.
As for Germany's problems, it has to do with how we measure value, institutional legacy, and obviously unwillingness to consider Nuclear.
Merkel was never really a pragmatist in the sense we would understand it - she was a political pragmatist, meaning really just 'populist' but more of the centrist type: I believe her Nuclear decision was mostly political.
But I believe people should be reminded from time to time that our world is pretty far from the best possible one: There's no particularly good reason that a child has to die from undernutrition every couple of minutes, that fusion research stagnated due to lack of funding, that people go bankrupt to pay for healthcare, etc. As a species, we're just too stupid and greedy to do any better...
If your case is that economic output is fixed I would flat out disagree. It's been shown over and over again that investment increases economic output. Which was very much the idea with Germany's Energiewende
Otherwise Venezuela would be the richest country in world history right about now, given their inflation rate.
It works identical to mathematical halving in concept. Yes you can keep going, and no you'll never reach zero. However every time you debase the currency to pay for something beyond your budget, you destroy some of the value underpinning it. You make the assets and transactions in that currency less valuable. You can infinitely subdivide the currency, and each time you do it, you destroy more of the value base. Eventually you're splitting pennies, so to speak. This is especially true in cases where you're printing to pay for something you can't afford (almost always the reason for such printing), so what you're really doing is eating the nation's asset base through the currency debasement.
Just ask Japan, they sliced more than 1/3 off their standard of living by debasing the Yen over the past decade. They had no choice due to their debt situation (they needed to stealth default), however it reduces the value of the assets that are held in Yen, making it easier to eg use dollars (or gold) to buy those assets (whether businesses or real-estate and so on).
So sure, in theory can you infinitely print currency. In practical application, you destroy the value of the things that underpin the currency and the whole scheme self-destructs given enough time and 'printing.' That puts a more realistic cap on actual potential money supply. The fact is, it inherently must be finite. Eventually your money is not worth anything, as you destroy the assets under it. As that happens, people stop accepting the worthless currency, it becomes too expensive to 'print' it and support it, and it becomes a self correcting system, in all cases and without exception.