it's not as easy as you wish, besides that I would prefer like you would describe, unfortunatly as long as there are borders/humans/whatever there will always be conflicts.
I don't see how you can claim defense is a useless industry while the world is watching a sovereign nation defend itself against an invading army.
$150 billion might not be enough, unless Germany plans to play no role in defending the eastern flank of NATO as Russia marches. They have been badly underfunded for decades, $150b is a down payment on catching up.
Putin's Russia is going to be in an economic and political swamp for a long time for this, it wouldn't be surprising if he chooses to move on to a new target promptly after Ukraine just because why not. The same crazy calculation that gets us here, it isn't too far of a stretch from there to argue (if you're Putin) for trying to take some of Finland's territory.
Germany needs to take this very seriously and I think they're beginning to.