Standard is a totally different mode. There's difficulties that have the same goal and set up as the lower difficulty, but with more hits and faster reactions....and difficulties in o!m don't have lower difficulties set up with varying hits, HP drains, and other stats at all. People totally don't make tons of difficulties for the same keys. What are you getting at?
You're not understanding a very clear fundamental point here, so I'm going to use other rhythm games as an example to show you how your idea of key-separating is entirely incorrect. I'm sure that a number of players here are familiar with rhythm games like Dance Dance Revolution / In The Groove -- 4 "keys", and 8 "keys", Pump It Up (referred to as PIU below) -- 5 "keys" and 10 "keys", and I'll also throw another game into the mix as well -- Technomotion, which is 9 "keys". All of these games have the SAME mechanic -- tapping your foot to the corresponding panel in accordance with the music, just like all of the key values of osu!mania have the exact same mechanic -- tapping your key to the corresponding receptor in accordance with the music. Here's where your argument fails: all of these games are played under this same mechanic, but have different physical construction to the play in question. If you have no idea what I'm talking about, look up all of these games and their respective modes. Fun fact: all of the previously mentioned games with the exception of Dance Dance Revolution, have large communities that sit down and make content for these games, much like the mappers of osu! and osu!mania work on content for this game. But enough about that, let me make my point:
Calling an ITG player useless because they can't play PIU is horrible and is a point that will likely get you in a lot of fire, because they are different games. Repeating this point with PIU -- you won't do well calling a PIU weak because they can't play half-double on ITG (6-key iteration), and you won't do well calling someone out on ITG double (8-key) for not being able to play Technomotion (9-key) much like you won't be able to get away with calling a Techmotion player weak because they can't play PIU double (10-key).
Calling a regular n-key player weak because another player is capable of playing n+1 key proficiently makes no sense at all, just like saying n+1 is worse than n+2 makes no sense, and yadayadayada. They are different games executed in entirely different matters, and if you take the time to do your research on the maps for each mode, you would likely find that 4K charts are constructed much more differently than 6K/7K charts. Most players who started on games like IIDX and transferred over here might be incredible at sustaining huge densities of cohesion in maps, but throw extremely high speeds at them and they likely won't fare too well without a ton of practice. Why? Because fundamentally, they are different games. It doesn't matter that they're all contained within one classification (osu!mania) -- what matters is how the game is played (4K/5K/6K/7K/8K/xK). You likely will notice that a lot of good 7K players don't land on 4K boards because they have no interest in playing lower key values, so you tell me: where does this double standard end?