Doesn't sound great: the message would be shown to the users, and there
are much more of them than the package developers. It will be a nuisance.
> I have a feeling that a lot of
> package authors choose MELPA because the barrier to entry is so low, and they
> may not realize how easy it is to get it into Emacs as well.
Try asking the MELPA developers about including that paragraph (or a
modified version) somewhere near the instructions for publishing
packages to MELPA.
Kaushal Modi