I'm a mid-core player. I choose to not play end game. (What's the incentive?)

[Ibi]

Anyways a horizontal progression system favors hardcore players much more and leaves more casual players in the dust.

This is absolutely true. Since equipment in horizontal progression is designed to have much greater longevity, the rate of acquisition is frequently significantly lower, meaning that players with less time to play (or who take longer breaks) fall behind much easier.

I always thought the definition of "mid-core" were people who dabble in endgame and will eventually clear, but aren't pushing it hard. I guess my no-clear scrub butt is hardcore after all?

There is no one definition of "mid-core". It varies (sometimes significantly) depending on the agenda the person using it trying to push.

Same for casual, for that matter, and you'll find many opinions on where the lower boundary of hardcore lies also.

let's not try to abuse "mid core" as the new buzzword.

I think that train left the station some time during Gordias.