I am with you on this. In addition to keeping all the own dates under my own control in this format, I make it a habit to routinely use this date format in non-consequential handwritten forms (guestbooks, permission slips, checks, receipts), to build awareness of this date format and help it "win".

My rule of thumb is if the input area is free text, and the benefit of my filling in the date is to the benefit of the form-owner, I'll use ISO-8601 dates and incentivise them to learn the standard. Join me!