I would venture a guess that apple/icloud accounts are the highest % account type amongst iPhone holders. All sorts of people have reasons for not having gmail / facebook / etc., but apple account setup is pretty prominent in device setup.
Sure, it might be the highest % among iPhone holders - but for a website, what portion of your users are going to be iPhone holders, and what portion of the Windows/Android crowd have Apple accounts?
You don't have to implement it to the exclusion of other methods. There are plenty of merchants on the web with Apple Pay and also Paypal, straight up credit card numbers, etc.
Of course you don't have to implement it to the exclusion of all others! But let's suppose that, say, 65% of users have a Google account, 75% have a Facebook account, and 40% have an Apple account - and the circles overlap such that adding Apple adds 10% to the total coverage. Now, if everyone who has an Apple account prefers it, and users with an Apple login yield 30% less revenue... gaining that extra 10% of users costs you revenue, because for every user you gained at 70%, three existing users cut their revenue by 30%.
