You don't need purchasing power if you do not intend to use it.

And if at one point you intend to use it, you will discover that its use may be heavily limited by blocksizess, OFAC complient mining or custodial restrictions, that won't let you out or into the state approved Bitcoin network.
 

 That's a separate point though. Economically speaking, hoarding the money commodity is not bad. It is a use.  

 Any (non-)activity is economic activity.

But any system also relies either on incentives or force. And as security increasingly relies on fees, excessive hodling while getting merged with state power means that at some point POW will be changed for POF (proof of force).

 

 At that point BTC may just become the fully integrated CBDC everybody made themselves delude to fight against. 

 Owning or doing anything incurs opportunity cost

There is no other human activity or asset where we call simply owning  and doing nothing with it "Using"

Owning a stock or a commodity or a piece of land somewhere 
and doing nothing with it is a speculative investment (ie it restricts the supply!)
its not a use of the thing.

*Particularly* since Bitcoin has security needs
( Which is the only reason it's valuable anyway )
but the holder doesn't contribute to those.

Nowhere else would we refer to this as "use"