I think it is wrong and highly misleading to claim that HaT, Waterloo 1815, Italeri, Zvezda have stopped production. They certainly have not. They have more or less stopped releasing new sets, but their sets are plentiful to be found all over the internet and they are regularily back in stock. I use figures for wargaming, one of the few using 20mm and for that purpose those manufaturers are very useful, since you can easily build British, French, Austrian, Prussian and Russian armies with their output. The same would go for people building dioramas I would think.
So tha they produce few new sets is something mostly the collectors have a gripe with, but for other groups they still do the use, and as a matter of fact, a manufacturer like Italeri that keep core sets for the hobby in print for decades is very important for new people entering the hobby, wargamers and the like.