I seem to remember a similar thread to this popping up before, and the response was much the same. I, too, am disappointed by the lack of plastic Plaguebearers thus far, and it has been the #1 thing stopping me from starting a Daemons army. However, let's take a look at things. Over the 18 or so years I have been playing 40k:
* Khorne's marines have seen 3 different eras, these ones, the monopose plastic kit (incidentally, some of the first models I got, and I still think they look half decent), and the current plastic kit.
* His daemons have also had 3 overhauls, starting with the muscled sword wielders, the axe-wielders, and the current plastic kit. Flesh Hounds have also seen 3 different versions in this time. Juggernauts have had the old CSM model, and the current design which itself has been seen in metal, plastic, and the Fantasy Chaos Lord on Juggernaut kit.
* Like all the Greater Daemons, his has had 2 versions, though I think he was the last to get his current version as they were released over a period of some months.
* Tzeentch has had the short end of the sorcerer's staff as far as marines are concerned, not having any official models still in production when I started playing, right up until the current kit was released (not sure what year that was, but around 2003? 2004?) Undeniably the worst off of the lot, as far as marines are concerned.
* Daemons have gotten more attention having had 3 eras of Horrors (the old pink/ blue balls with arms & legs, the scary 'changing' ones, and the current plastics), as far as I can remember Flamers have had 2, unless I'm forgetting any... Discs had no official model when I started, right up until the became the current Screamers, and have appeared as various mounts for characters in recent years.
* Again, Greater Daemon has had 2 versions, being the 2nd to get his current model. Only one of the 4 to have a special character model made of his greater daemon.
* For marines, Nurgle has boasted 4 versions - these guys, the monopose plastics (again, still quite like these guys... probably prefer them over the current ones), the metals from the same era, and current kit.
* Daemons have seen only 2 versions of Plaguebearers, though the rumour is to expect new plastics soon. There have been 3 different versions of Nurglings, and 2 versions of Beasts, albeit with a large gap between them with no model available.
* Again, 2 versions of the Greater Daemon, being the first one to have the current model released. Also the only god to get his own Daemon Prince model.
* The ol' Noise Marines have been through 2 versions, starting with these guys, and then the current models.
* Daemonettes have been through 3 versions, these, the Diaz ones, and, sadly, the current plastics. Fiends and Seekers/ Steeds got their current models after a lengthy absence from the range.
* Same old, 2 versions of the Greater Daemon, IIRC being the 3rd to get his/her current model.
So, looking over that common ground, and ignoring things like one off special characters, etc, it would seem Slaanesh is worst off... having had more Marine models than Tzeentch (only just), his main Daemons having had the same number of treatments as the rest, but his specialist daemons lacking for most of the time. Nurgle, on the other hand, has had the most releases of marines, with more daemon releases than Tzeentch, but less than Khorne.