I did a quick search and SWIFT has 11k member institutions who move $5 T daily, in 45 M daily messages. So let's apply some Pareto here and let's say that 20% of those messages do 80% of the value, and we have $4 T in 9 M messages to call "high volume" anything above $444,444 current purchase power.
How many institutional and non-institutional actors are sending $444k per transaction DAILY? I doubt there are one million, but that would be the Bitcoin network user numbers you're looking to make the initial calculation in the first post.