Games Workshop since the 1980s has seen a growth of army book straitjacket, or codex threadmill, and a loss of freewheeling creativity. The format of producing extensive and growing army ranges made it harder for the studio to follow their fancy and jump on odd one-off releases where a couple of handful of sculpts in some cheap metal moulds sufficed to call it a day. Increasingly, the miniature releases turned ever more rigid into catering to the codex threadmill. The transition into full plastic ranges only exacerbated the army book straitjacket, since hard plastic moulds are so expensive.
This trend has been something the studio has always fought back against, as if they wish to recapture the freewheeling creativity of the 1980s. The 1990s saw a healthy number of Regiments of Renown, petering out with new iterations of Mengil Manhide's Manflayers and Ruglud's Armoured Orcs after 2000. Niched vignette pieces (e.g. animosity Orcs), summer campaign releases and things like Specialist Games and Dreadfleet all stand as proof of attempts to have an outlet for freewheeling creativity in niche areas. As do Forgeworld itself.
Yet the army book straitjacket was inevitable. To sell well, most releases had to cater to existing armies, or had to introduce whole new armies with extensive ranges (Tomb Kings, Ogre Kingdoms, Necrons, Tau, Dark Eldar). More exploratory half-sized new armies were repeatedly attempted up to the early 2000s, with everything from Sisters of Battle, 1990s Chaos Dwarfs, Kislev and Daemonhunters, many of which turned out to be neglected one-offs in the long-term codex threadmill.*
Gone were the days when Citadel could release a Nipponese rocket launcher with crew and call it a day. Things had to increasingly fit the big army books.
The 40k Imperial Guard range serve as one example of how GW's freewheeling creativity was stymied over time (though it is not an example of peak freewheeling creativity in the 1980s):
The 1980s Imperial Army was standardized, all Necromundan if you so like. Plastic and metal.
The 1990s Imperial Guard sported plastic Catachans and metal Cadians, Mordians, Pretorians, Tallarns, Valhallans and Steel Legion in 2000. Lots of different regiments to hint at a vast setting with infinite variety.
The 2000s Imperial Guard sported plastic Cadians and a brilliant spasm of metal Vostroyans. There was no shortage in the early 2000s of new alternative Guard regiment descriptions and artwork, yet without models. FW also produced Elysians and Death Korps of Krieg.
The 2010s Astra Militarum was all plastic Cadians and Catachans. No White Dwarf exploration of other aesthetics without miniatures, since that could throw third party manufacturers a bone (IP mania is destructive for creativity).
Likewise, it may be noted that recent plastic kits' inclusion of funky details like silly Nurgling minions, fly mutant Terminators or a scorched heretic for the Sisters of Battle is a way to do some fun niche stuff within the constraints of all plastic ranges. As is the tendency to more carefully (and less freely) pose plastic miniatures like you would a metal model, but previously not a multipart plastic mini.
As such, when you see funky old models making a return in Necromunda or niche box games, remember that the army book straitjacket was something the design studio always tried to break free from, seemingly to recapture some of the exploratory and freewheeling creativity that was a hallmark of Games Workshop in the 1980s.
It should also be remembered that while ranges of primarily metal or resin sculpts allow more creative freedom for the sculptors, ranges of multipart plastic allow more creative freedom for the hobbyists. It's a trade-off, and no kit has ever struck a perfect balance between the two. The plastics released around 2000 were the peak of convertibility, but their poses were not as naturalistic as those of better metal models. The current trend of virtually pre-posed plastic kits is clearly an attempt to recapture some of the sculpting quality lost when moving from metal to multipart plastic. Yet it occurs at the loss of Lego-like customizability for the hobbyist.
Just some observations on GW creativity through the decades. The spark has never died through all the natural style shifts, but the constraints have increased. That is one reason as to why the 1980s was such an outstanding creative rollercoaster. :)
* Several newer half-sized armies in 40k and Age of Sigmar seem to have pulled off this stunt with more success, such as Harlequins and Custodes. Plastic sticks better. Change in CEO aside, this is one viable way for freewheeling crativity to explore niches of the setting with miniatures. It's still an army, but the required work and investment is more limited than entirely new fully fledged armies require.