upstart was always a mediocre to terrible sysv replacement. The fact it couldn't handle such insignificant programs like postfix natively speaks volumes.
> When I wrote "replacement", I didn't mean that all sysv scripts must be replaced. Postfix sysv script works fine with upstart, doesn't it?
I'm honestly not sure if you are making fun of upstart or if you're trying to say that having to keep a sysv compatibility mode around because upstart is not able to handle an extremely common daemon is somehow acceptable.
The 'compelling' reason was "sysv has a ton of problems". Upstart was an earlier attempt to resolves those problems - whether it does those well or not doesn't mean that there wasn't a compelling reason to create it.
Edit: Upstart was a great sysvinit replacement, at least during those years before systemd appeared.