I feel like I want to agree a lot of with this... but I can't help but feel that the examples chosen conflate --accidentally/incidentally or not-- mere "grind" with "difficulty"... or even the openness or longevity of "expression" (skill-wise or otherwise) I personally think might be most pivotal to something being "midcore".
Or, I would agree that developing "midcore" content will require some additional, deliberate investment on the part of the devs --though not necessarily in terms of separate content-- but think that being "midcore" is separate from and agnostic to the amount of time something requires to progress --especially in terms of getting to the meat of its gameplay, of all things. To my mind, "midcore" content should not necessarily require substantial "investment" from the player just to access the bulk of that content's span/depth of potential engagement.
Simply put, the kind of "midcore" content I'd most want doesn't require altogether different content types. It really does just require more scalability out of the core content... and perhaps a bit greater breadth of optionality in surrounding systems -- say, within matchmaking options, as to better curate one's experience without necessarily running through the hoops, mutual exclusions, and reduced relevant player pools of PF, etc..
For instance, I would much rather have option to do slightly reconfigured "Savage" dungeons themselves --perhaps even with a low-ish max ilvl and weekly-cap drops/chances-at-drops shared with Savage raids-- than a wholly separate (and imo, visually-degraded) form of dungeon a la Criterium. I would rather see Relic grinds or reinvigorated Wonderous Tails analogs introduced earlier complete with rotating bonuses to shorten queue times and simple toggles for taking the given content more seriously, not, or either one, etc.
Personally, the most enjoyable examples of what might broadly be considered "midcore" content that I've experienced in this game has had is rapid-queues late-night Extreme PuGs during Light bonus rotations where one would end up seeing many a familiar face, with many people playing quite well for the fun and speed of it despite not being wholly forced to play at that level, without any run feeling outright threat-less. Oddly enough, original Diadem also came closer to that for me than Bozja, if only because of the far more immediate optionality it gave compared to Bozja or Eureka before it. Elsewhere, it'd be the likes of M+ if matchmaker-capable and allowing one to pick which combinations or spans of dungeon, level, and affix alike they want to queue for (with only some soft incentive to vary things up rather than being locked to this or that) with whatever chosen constraints in matchmade party and more functional content-specific commendation and a handy "Stay as group" feature for after a given run for those who want to keep up the fun momentum.
Such not only provides things that reward dedicated play at whatever level of performance with a sense of progression but also further leverages things that reward personally high levels of play and progression in terms of one's own skills/performance/etc. by reducing barriers to entry/engagement in time and hassle while allowing for more interesting chained experiences. And that has at least as much to do with quality of surrounding systems as it does content type.
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That said, to give a rather different example, despite liking the ability to take content piecemeal to better fit in with my hectic schedule, I like when raids in (at least their original and designed-for form) feel like cohesive and complete experiences. Here, Alliance Raids, imo, do that far better than the actual so-called "Raids". But why does it even have to be all one or the other? Why can't one queue into the whole Normal Raid "series"/"tier"/"wing", if they so please, complete with (interesting, for a change) trash with strong tome rewards or the like (perhaps increasingly degraded after the first clear of each segment for each week) and flavorful, immersive environments for that more cohesive and "raid"-like experience? Allow people to queue specifically into the fight they want to farm for their weekly loot as well, of course, but why not start from the whole and then offer QoL via divisibility rather than just treating any content capable of more difficulty as locked in stone to mere boss arena series?
Given that, perhaps if the likes of Bozja didn't lock their more entertaining portions so deep behind grind that is typically varied only in shallow visuals (and is therefore all too often mindnumbing due to the impossibility to create interesting and choiceful tuning points against the provided progression systems), I'd hold fewer reservations in agreeing.
But, I do think more immediate breadth of choice is key, too, not just the fact that something will ramp up over time. Else, "midcore" is a transient personal sweet spot over which the player has agency by which to keep themselves in as long as they desire.