I'm confused as to why multiclassing is the underlying problem here. It very much feels like we're making an intrinsic argument that dips in and of themselves are bad, not in the pursuit of some particular design reason.
If a Fighter 1/rogue 7 is better than a rogue 8, that suggests whatever rogues get at 8 isn't particularly impressive. I don't think we can evaluate higher level class features as generally "worth" putting up with less impressive mid-level features, because it's very much not a given that any game ever uses them. With that in mind, I think it's much more pointing to a need to make class features after level 3 more compelling than the 1-3 features of other classes.
Honestly, I don't think there's much wrong with a lot of the classic dip options. Hexblade, for example, isn't really that overwhelmingly powerful; you're just adding "attack with a weapon" to the most of spellcaster action options at expected accuracy. Given that it's an action cost vs. casting spells (and excludes clerics and wizards), I think it's honestly a better argument that "attack with int, wis or cha" should probably be a feat instead of requiring multiclassing.