If everyone start storing bitcoin and there is no single transaction for days,all miners will quite mining as without transaction there would be no reward.
Will it not kill bitcoin and make it useless?
It can kill but this scenario is not going to happen because there would be less opportunity to use bitcoin and it will not be called bitcoin anymore.
We store bitcoins for a long time but we do not have that much power that could affect the market or the bitcoin price so this is directly depended from big holders, big traders and big exchanges.
I think that bitcoin addicted people hold more bitcoins that others, and we should be sure that exchanges and bitcoin mixers have a large reserve and should count that as a storing/holding too.