Yeah, it's pretty clear by this point that doing restic instead of plastic is an economic decision, not a case of it not being possible to get plastic produced quickly.
As Nightsword said a while ago, it was a choice between "Corporation marines plastic sprue", and "Entire Corporation range + Entire Veer-Myn range + Project Pandora" as far as Mantic's production budget for that 3-month window went.
With Mantic's initial elves and undead, it's possible that all sorts of other factors were at play... high startup capital for the business, Renedra possibly offering a reduced rate in exchange for big multi-sprue contracts since not many people were using them back then. We don't know. VAT rise of 5% is definitely a concrete factor that has raised the price since the early days.
We'll probably see a return to plastics (in a big way) in a few years, whenever Mantic is big enough to drum up enough capital, and the need to "get loads of stuff out there" cools down a bit.