Strictly speaking, of those three only upstart is actually a replacement for System 5 init. OpenRC and daemontools run under an init, and by and large replace System 5 rc. There are two toolsets (soon to be three) in the daemontools family that provide a replacement for init: nosh and runit. daemontools itself does not, and has had more than a decade of people adapting it to run under init replacements because the vanilla system only targets late 1990s inits, including under upstart as a matter of fact.
From one pedant to another, thanks for the correction, and the informative links!
So, I guess that the Debian folks were incorrectly claiming that they were looking for a replacement init. They were looking for a replacement rc system, and were willing to accept a replacement init as part of the package.
* http://homepage.ntlworld.com./jonathan.deboynepollard/FGA/da...
* http://homepage.ntlworld.com./jonathan.deboynepollard/FGA/in...
* http://homepage.ntlworld.com./jonathan.deboynepollard/FGA/sy...