I checked my logs of the last 7 days. My mail servers have sent emails to
29.401 different domains worldwide, 29.015 via MX records and only
286 via A records, which is less than 1 %. For a statistik about how many
emails are sent via MX records compared to A records the result would
even be lower.
The missing MX records for the 286 domains are the result of lazy or
clueless mail administrators.
While I think that deprecating A fallback would be a good idea, I don't
buy that admins currently using it are lazy or clueless. What they're
doing is explicitly permitted by the current standards.