The downsides are millions of dead people. There is never a "net positive" situation here.
Yes, there are many downsides to war, those millions of dead people you mention amongst them. The thing is, just because you have decided the downsides of war - millions of dead etc - do not weigh up to the potential upsides does not mean your neighbour thinks the same. Eventually you'll get a neighbour who sees you as ripe for the picking, unprepared for war as you are. Some people might die, sure, you being one of them, but in this neighbour's eye the trade-off is worth it.
Humans are a belligerent species. Just wishing for this not to be the case does not make it so.
[1] https://en.wikipedia.org/wiki/Si_vis_pacem%2C_para_bellum
On the other hand kinetic bombardment is much better than than nuclear exchanges for the people left alive and makes nuclear weapons pretty close to obsolete.
It’s like nuclear weapons.
You cannot reverse or stop technology from progressing and you can’t stop governments from using that technology to build weapons.
How many wars did it take for humans to adopt a different “steady state”? And even now it’s not a real steady state.
Maybe we have more lessons (I.e mistakes) to learn from in space where billions die. Hopefully not. Hopefully we let the 20th century continue to instruct us.
Meanwhile world leaders spend well over $1 trillion a year on their militaries. What else could we do with that much money?
There’s absolutely no reason to launch humans to space for the purpose of operating space-based machinery.
The current us space force has the sole purpose of supporting ground wars. And when they start battling in space in a hundred year, they will nuke planets or throw rocks on them.