Ah, this age-old discussion.
Indeed, the lack of incredibly difficult maps is the main reason osu!mania may attract players from other games, but can't make them stay. You mention Imperishable Night 2006 being the sole ranked boss map at the moment. From what I can tell, the 7K Lunatic difficulty on that map is a carbon copy of
the Overjoy 2 BMS chart, except with a sprinkle of LNs in the middle. (Which tells us that BATs in this game are totally fine with ranking imported maps from other VSRGs... but that's a rant for another day!) This means that, if they really wanted to play 7K Lunatic, they can download the BMS file and play it on Lunatic Rave 2 or other BMS simulators. There is little justification in calling it a "boss map" when such high-skilled players can find a better alternative.
Of course, the rest of the ranked osu!mania mapsets look - and play - pathetically easy compared to IN2K6. The most difficult set I can think of is Shuffle Heaven, and even the SHD difficulty on that map has players capable of FC-ing, maybe even SS-ing it - implying that the map is still in these players' comfort zone, and nowhere near the challenging, stamina-draining, eye-twisting zone that we want to see. I'm honestly disappointed in how painfully easy some of the latest ranked sets are. It seems as if mappers are only meeting the bare minimum of the ranking criteria, just to attach a scoreboard to their amazing maps where "every player can expect a certain kind of standard when playing [it]".
However, once we start allowing ranking of actual "difficult" maps, I fear that mappers will completely change their attitudes toward mapping, and start creating new maps that are absolutely ridiculous and physically impossible to play. You linked three of Entozer's mapsets, all three of which, by definition, do not possess a full spread in difficulties. Now, let's assume any map ranked under "Approval" can bypass some of the ranking criteria, namely the full-spread requirement. Why would anyone map a full 4-difficulty mapset, when they can make one stupidly hard difficulty and get it approved? I believe
this happened some time ago in standard. What this ends up causing is maps with one absolutely insane and expert-level difficulty. Think about it from a rookie/amateur's point of view. There is this one song that you love, and you recently found out that it has been ranked on osu! - only to find that the difficulty/-ies are way beyond your play level. Pretty disappointing and discouraging.
On the other hand, these monster charts actually take quite a long time to create. This is not standard, Taiko, nor CTB. This is osu!mania. Compared to other game modes, we have seven possible positions of notes per timing measure.
Theoretically, they take seven times longer to create, compared to monster maps in other game modes. (I reiterate: theoretically. Actual time taken would be around three to four times longer.) Already, you are looking at quite a hefty sum of work for the mapper. I don't know how much of the ranking criteria these approved maps will be able to bypass, but if these maps still need a number of mods before officially being given a scoreboard, that only adds to the total time and effort required for all osu!mania players, mappers and modders alike.
It's a dilemma, really.
If you want to appeal to the beginner/amateur players, rank such maps ONLY if the entire mapset contains a full spread. No one wants to have their favourite song spoiled by some stupidly hard map.
If you want to appeal to the higher skilled players (in other words, make them STAY, and potentially influence them to bring other monstrous players), rank these difficult maps under any circumstance.