Agreed 100% here. The important aspect to me is that character doesn't get new features past a certain point, except for those feat choices I make myself. Sort of like the MMO idea of "the game really starts once you finish leveling". Now my character only progresses by achieving actual events in the game, which is a feel I really like.I think you and I must view E6 differently, because “a powered down version of regular advancement” is definitely not the motivating idea of E6 as I see it. The point of E6 isn’t to slow advancement but to halt vertical advancement entirely past a certain point, while maintaining horizontal advancement. It’s shutting off the level treadmill but still offering expanding character options. “Leveling up” stops being about accumulating more power and becomes about refining your expression of your character.
Plus, I think it's easier to add new feats and new items/boons to the game to address any holes in horizontal progression than to have to plan out alternate advancement for every class.
I'd also say E5 is a much more natural stopping point in 5e than E6. It was only E6 in 3e to ensure all casters got their 3rd level slot; 5e's homogenous progression means that fix isn't necessary. It also makes Extra Attack and 3rd level spells a natural capstone, missing out on these makes any dipping a much riskier endeavor. E5 also serves to balance out subclass access; many but not all classes get their T2 subclass feature at level 6, while at level 5 every class just has gotten their T1 feature or features, no class gets a subclass feature at level 5.