Usage only matters if people are holding. Take this, for example:
We have 100 BTC. There are 100 people interested. All 100 are buying and instantly selling their BTC.
1000 more people get interested. The market is constantly buying and selling, so they jump in as well.
This has no effect. If people HODL, this will change things. But that's just as easy to do with 1k people as it is with 1m.
The intermediary doing the conversions is a HODL'er.
They have to hold reserves in the asset pair in order to operate.