I'd love to know what proportion of the Bcash spent through them was the free forked coins versus coins that someone actually bought with their own money.
I very much doubt that people investing in bcash are going to spend it. BitMEX pointed out that bcash mainly functions speculative object, more so than any other coin, where even Roger even admitted that.
The people with free coins could either choose to cash out, or spend, where the latter seems more likely and more favorable to avoid paying tax. Spending crypto in most countries means you're exempt from paying tax.
If you are really curious and into blockchain analysis, you can always allocate time to roughly figure out how much bcash is being processed through BitPay. If people spend hard enough they'll run out of coins in a year or two.