Why not? People keep their wealth in a store of value (e.g., fiat currency savings, bonds, stocks, mutual funds, real estate, gold) and then sell those items as they need that wealth for spending.
Because with all of those, there are no miners that are needed to support the network. If I hold gold, nothing external will affect my gold holdings. But if I hold bitcoin, and mining reduces dramatically due to low fees, then the security of my bitcoin is affected.
A bit of a tangent to this thread, but according to this paper transaction fees by itself are not sufficient to support the bitcoin network:
https://www.cs.princeton.edu/~smattw/CKWN-CCS16.pdfThis may seem intelligent, but it's not necessarily true. Even if people are mainly hoping to hoard, it would still be traded in a relatively liquid market, and there would be a fair number of exchanges between BTC and fiat currencies or between BTC and gold.
How do we know it would be enough? If it is only a store of value then most people are going to be hoarding. A tiny percentage of the world's gold is actively traded compared to the amount in existence.
The main reason why BTC could not be used as a store of value in the long term isn't really to do with miners - it's mainly to do with utility. Any cryptocurrency can have utility, so Bitcoin doesn't inherently preserve its value if it doesn't have utility.
This is a key point and another reason why bitcoin cannot survive as a store of value only. If another cryptocurrency can function as a transactional currency and a store of value, then people would flock to it instead of bitcoin.