You're starting with the assumption that everyone would be using Bitcoin purely as a store of value. This, in my opinion, is extremely unlikely to happen on such a large scale that it woulds massively impact the number of transactions being sent with Bitcoin.

Even if Bitcoin was mainly used as a store of value, you'd still have people using it either for short term or for long term storage.
If used for short term storage, there would still be enough transactions to pay for mining fees.