Just because you don't play 4-key maps ever, doesn't mean that others don't. As mentioned before, it is statistically more popular. Regardless of any sort of extenuating circumstances that you have had that stopped you from being able to play, there is a large demographic of players that do.
The consideration of you playing other games makes most of your argument irrelevant as well, because you're already implicating that osu! isn't your main game of choice. To that, I would normally say that it's nobody's problem but your own — however, I do understand the issue with >4k maps not being as prevalent, and it's something that is being addressed.
Think about this a bit more rationally with more accurate figures here: these are the number of beatmaps that are currently ranked, according to osu!search, for each particular keymode.
4k - 658
7k - 364
anti - 5k: 69 / 6k: 38 / 8k: 46 / 9k: 14
total: 1189
The problem is not nearly as apparent as people make it out to be in terms of content. Yes, 4 key does have a substantial lead at ~55% of ranked maps, 7 key has ~31% of ranked maps, so the content is out there. It's clearly not as prevalent. Beyond that, it is up to the players to make content for the game modes that they want if they're not seeing what they would like to play. Content creators go out of their way to make content for given modes, but if you feel like you're not seeing enough of that mode, then learn how to create and curate content by yourself and/or with other people.
You also mention this concept of "fundamental core elements" and mention that 4k and 5k feel similar, and how 6k/7k/8k feel similar, and I'm able to tell you both from personal experience, and from others around me, that most players who are trying to get into different keymodes after playing previous ones struggle with odd number keymodes more than even number ones because of the disbalance required to be successful.
Even in that regard, you're still wrong because there are multiple core elements in any VSRG, and osu!mania goes a bit beyond that by combining elements from multiple games. Timing is stricter here than most games. Release timing isn't very prevalent in other games. In addition to that, you've got varieties of map styles - speed maps, stamina maps, technical maps that test multiple elements, long note maps, maps that play with SVs, there's a ton of content in there, and such content exists in both 4k and 7k (and if you're denying that, you're not looking hard enough, plain and simple).
This selection, as mentioned before, will expand to multiple keymodes. With 30-40% of the selections being intended as 7 key alone, more content should be filtered into the game that people might want to play. And if for any reason it doesn't, and I'm still heading up the creation of these lists, I'll adjust them accordingly to bring more attention to that mode, because I would agree that it is in more dire straits as a main mode.
While irrelevant, score v2 has been worked on between and throughout tournaments, and the effects have been clearly noticeable, again, if you're paying attention. The weighting of scoring that was a problem in the 2016 MWC was addressed, rainbow judgments actually have meaning, combo has a better definition, scoring is overall improved and more accurate.
Long story short, you're mentioning that things aren't being approached, but the very existence of this ability to pick through maps within a list again is in itself an approach to a problem with the loved system that users were seeing. Take things one step at a time and solutions will eventually arrive.
, well I hope at least once this map (https://osu.ppy.sh/s/332673) will get its chance... 
