> If software does not include itself a mechanism to use an
> alternative "transport" then one would assume it _requires_ the real
> thing.

Why would one assume that?  How would it tell the difference?

> I can't think of any package that does not allow one to specify the
> mailer at the very least as a compile-time option, in many cases it
> is a run-time parameter.

So, if my host uses postfix instead of sendmail, I have to
individually configure /usr/bin/mail, emacs, every MUA installed on
the system, inews, INN, apache, and so forth all to use it?  This way
lies madness.

Unix never provided anything better than /usr/{sbin,lib}/sendmail as
an interface for injecting a mail message.  So that's what the
interface is.  Now we have to cope with it, like it or not.

(Incidentally, since I don't think anyone has stated it strongly,
modifying files or symlinks in /usr/sbin/sendmail is not an acceptable
way for this configuration to happen.  /usr might be shared between
machines which want to use different mailers.  If we're going to use a
symlink in lieu of a config file, the symlink needs to go through
/etc.)