I would wager that the vast majority of people consume email via Gmail, Outlook, or Mail (Apple). You might have the holdovers like my parents who still use Thunderbird (because in early 2000's that was ideal of POP3 accounts, and they're used to it, and that's just how it will remain). All of those clients display HTML by default, and all marketing / account-related / branded emails are HTML emails. Every single reason cited in your link for choosing plaintext are irrelevant for the overwhelming majority of internet users worldwide. We could definitely productively debate the ethics surrounding marketing initiatives and data privacy, but at the end of the day, to me, plaintext destroys most of the functionality that makes email so useful.