40K art has kind of stagnated, really. Like at it's heart none of it's really supposed to be literal and diegetic, and instead it's about translating vibes and evoking a general sense of inscrutability, decadence, and archaicness. It hinges on this sort of gothic cathedral look to do that, which does convey that vibe, but it really shouldn't only be relying on that specific aesthetic to do it, and should draw from other old architectural styles too.
For example: the spaceships are figuratively "cathedrals" but probably don't diegetically look like medieval european cathedrals even though that's how the art portrays them, so it would be a huge step up if they made them look like ancient temples from other cultures too and just kept it so that core design was standardized but all the aesthetic bits varied by forgeworld or sector to represent how they're all grandiose and archaic but aren't literally all just copying specifically 16th century French architecture.