1) The game DOES have mineral storage. If you haven't figured out the "Queue up 100,000 battleships at a single spaceport and cancel / add to the queue as needed" trick after 11 months, well, idk what to tell you.
2)
this is a dumb idea tbqh
if I actually need the resources I'm spending them long before they can pile up to sizable quantities
if they actually are piling up I don't need them and won't miss them
This is the correct answer.
There are two economic states in Stellaris: you're either building fleet / infrastructure so your red mana goes out of your treasury the moment it comes in... or you've reached post-scarcity economics and you'd barely even notice if 20K just vanished because you're gaining 2K a month already. In the first circumstance, you have no resources to lose; in the second circumstance, you don't care if you lose it. Thus the suggestion has failed to achieve its objective.
The OP isn't
entirely without merit, because I agree with his starting premise that at the moment in Stellaris "
Losing a planet means nothing" (except possibly in the very early game).
But I am of the opinion that increasing the MATERIAL consequences is exactly the wrong way to go about trying to solve the problem.