The scenario is far-fetched
... in that you need to find someone with an Apple Watch that actually uses Apple Pay. The best thing I can think of to say about the latter is that it's proof that Apple users won't use things just because they're told to.
"Eleven per cent of credit card-owning households and 66 per cent of iPhone 6 owners in the US have signed up for Apple Pay, four months after it was launched [...] However, issues like low repeat usage ... are hindrances in its success. [...] 48 per cent of users have paid with Apple Pay just one time" — Mobile World Live
"More than 95 percent of iPhone 6 and 6+ users who could have paid with Apple Pay on Black Friday didn’t [...] Five weeks after the launch of Apple’s revolutionary payment method more than 90 percent of these users hadn’t even given it a try." — Pymnts.com
"It’s no accident that numbers on actual Apple Pay usage are hard to come by. As a percentage of total sales it has to be inconsequential." — Forbes