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?
Absolutely not, hoarders are what give bitcoin its value, bitcoin would have no value what so ever if everyone tried to give it away for other stuff. This concept is hard to understand at first but Holding savings in bitcoin is vital to the network. You should listen the youtube link i will post below, it explains it perfect.
https://youtu.be/e1eKHXdlx2Y?list=FL_1xDQgnywzy4JeHeRy1eKARoger ver pushes the whole bitcoin should be spent thing and then on the other hand admits that hodlers give bitcoin value

It is true if the person hoarding Bitcoin purpose is to increase the price. But, if all owner store Bitcoin (not spending it even once) will posibly make the price increase for a while but since the idea is not spending it, then this demand will get tired and be pulled out to use for another / other market. From here, Bitcoin price will decrease until the time that Bitcoin demand will be gone making its value to become worthless. (Though it would be impossible to not sell if we see Bitcoin price is very expensive.)