hold on! lets have a look at this shall we?
Troops
we have Cadian and catachn in plastic, Tallarn, Valhallan, steel legion, vostroyan and Mordian iron guard all in metal. okay well you can argue that they are suplemtary but how about our specail weapons? you know those things that every vets squad takes 3 of? yup metal blister packs.
HQ
Straken - metal, Advisers - metal, Techpreist - metal, Primaris psyker - metal, Pask - metal and with Creed, Kell and Yarrick in fine cast
others slots
actauly not too bad, just Ratlings, Wyrdvane psykers in metal and no Rought Riders to speak of. (Vendetta being a forgeworld resin upgrade kit)
if it wasn't for the fact that special weapons are still metal blister packs containing 2 different weapons that depend on the faction I might have let it slide, but it is still nearly impossible to build a Guard army that does not contain metal, finecast and plastic models. also you mention the age of some of the chaos models as a point against them but you seem to be forgetting that the basic guardsmen are just as old.
I'm not trying to say Chaos doesn't deserve some new models at some point but Astra Militarum are far from being all plastic as well (because someone decided what we needed was new betterer twice the cost ogryn and a vehicle (taurox) that offers almost no advantages over the existing Chimera)