I feel the space marine change is much like AOS. I'm not sure how much is still made, but I don't think I've seen a regular marine promoted for a long time.
Just like much of Warhammer fantasy battles. However, I think WFB were more promoted than classic marines due to AOS.
Point to some modern classic marines promotion? None.
AOS still, or at least up to recently to 2 years ago, had WFB figures in the regular AOS armies.
So 40k actually has some worse white washing going on.
Apparently a lot of guard have disappeared also.
One issue with a TOW type game, is it has to be set in another time or place, but still fit in.
This should be easy.
Tyranids and marines have both changed drastically after 2000(?). Necrons too. So a game set after HH, but before the modern setting is easily possible.
Classic plastic nids, marines, probably necrons (could still offer metal like they do).
Even with plastics, they have all changed drastically since then, style wise. So a 90s-early 2000s game could hold its weight.
Basically 3rd edition, I guess.