Plain text email continues to work just fine for me every day.
I don’t know what client they are using, or if they never received a properly formatter reply in their life.
I am the one oddball in my office who doesn't use Outlook and who sends plain-text emails with ">" prefixed quotes. But I'm under no illusions that anyone else is going to be convinced, and I no longer make any effort to try.
For decades, inline replies worked perfectly—you'd quote the relevant part and respond right underneath it. But now email apps are "helping" by trimming messages into compact views, cutting off replies right at the first quoted section unless someone taps "show more."
I've basically abandoned inline replied and have gone back to dumping everything at the top like it's 1995.
The irony is these apps think they're making email better by hiding "clutter," but they're actually making conversations harder to follow.
Kind of like how one adjusts their language / choice of words / choice of topics based on their recipient.
The easiest way for this crusade to succeed would be to take aim at Outlook and Gmail and try to make them change defaults.