You bring up a great point about how they affect playstyles, and I think it's not just marines that are victims of this. Recent years have really seen GW pushing "centerpiece models" that they want to be almost essential to play a faction on the tabletop (Silent King, Norn emissary, Lord Solar, etc).

Between that, eviscerating wargear/loadout options, & a general focus on "competitive play" 🤮 it feels like there's way less customization/variety in army lists.