Isn't that just the new standard set by phones and instant messaging? Iterative instant commutation rather than "letters" of formatted text. I too write a lot of messages in a single pass and press *send*, only to "edit" any errors afterawrds. (or a with good old "*afterwards" message afterwards)

Phones are the new default, but they are terrible for long form writing. Moving a cursor to the middle of a section or moving a section around is frankly more effort than its worth. Just send it and clarify later. I'm not saying its a good thing, but i would call it more of a shift than a decline. If you ask them to write a report the writing may spectacular, since they're actually by a proper keyboard and in a format with more strict writing standards.