Sending value is only one reason to use BTC, it might even be the main reason. However, BTC is a ledger for posting vbytes on the timechain. That's all. Those vbytes can represent locking and unlocking scripts to "send" sats. But those vbytes can also be used to establish payment channels (Lightning Network), "anchor" data from altcoin chains (Liquid Network), put messages in blocks ("Chancellor on the brink..."), host JPEGs of 8-bit monkeys (Ordinal Transcriptions), or JSON objects that represent token transfers (BRC20) or even keeping botnets online. You can even use up a ton of vbytes doing wallet maintenance (UTXO consolidation a.k.a. self-transfers) or doing fancy script 'programming' onchain with miniscript. It also cost exactly the same to send 1,000 sats onchain as it does to send 1,000 BTC (assuming it's just one UTXO input, single sig). Therefore, every use of a vbyte does not necessarily correlate to any value changing hands. It certainly does not matter how you choose to denote that value (USD exchange rate).

As for the addresses: unique addresses do not identify unique users. A user can have many addresses. Further, a single address might be shared by many users (in a custodial or multisignature relationship).

Since the following are true:

...it follows that USD transaction volume and new address use can decrease while fees remain flat.

No. We use BTC to fuck fiat and make it obsolete. This is bullshit fiat mindset with "sending value over bitcoin network", is just a replacement of paypal, but still using fiat. Stop thinking like that and remove slowly the word USD from your vocabulary. We are using only bitcoins / sats.

It's not just to send value, it's to hold value. In other words Bitcoin is an asset thats primary use does not have to be as a medium of exchange for accessing the blockchain as a payment rail.

The amount of USD is different from the amount of sats that are sent daily, they are not interchangeable as your usage of the word "or" would imply. If each sat were worth $100 one day and each transaction would send thousands of sats, does that show less demand than today where people still send sizable denominations of Bitcoin?

There are also more ways to transact using Bitcoin than only making on-chain transaction (see the lightning network). And, to the chagrin of some who thought there would only be monetary use-cases for the Bitcoin blockchain, there are also people who use the blockchain for its immutable properties (see ordinals and inscriptions). These allow for individuals to make low value transactions with high fees (in which case dollar amounts would be a poor metric for measurement).

TIL

I had just assumed. My rage has now subsided.