I'm not surprised that very few people here would agree with C speeds, and it's not something I'm terribly concerned about.
Part of the reason I'm more or less fine with how things are is that you have a wide range of speeds available. I don't know how high the current settings allow you to go, but I think that for a BPM of say, 80, you can adjust up to 20+ if you are comfortable at those speeds, and yet for something with an extremely high BPM you can drop all the way down to 1 if you want. This gives a much wider range of speeds than a set selection of multipliers. With this setup, it's possible to keep your scroll rate relatively close to the same value by adjusting in smaller increments.
As an example of where multiplier speeds would be annoying, take the difference between 150 BPM and 160 BPM. At a multiplier of 4x 150 BPM becomes 600. If 600 is your ideal speed, and you wish to play a 160 BPM song at as close to that speed as possible, you are stuck with either selecting 4x for an effective rate of 640, or 3.5x at an effective speed of 560. If you are like me, and small deviations from your ideal speed can cause large differences in your score, this can be a big problem. And it was, since o2jam used this system. There were many songs with BPMs that were not comfortable at any of the available speed multipliers. Your math does little to change this particular problem, since the effective fall rate may still lie at points where the multipliers cannot bring you close enough to your ideal rate. This problem becomes even more pronounced at higher BPMs, where the difference between a multiplier of 2x and 2.5x can be very large. At 400 BPM (effective rate after your math) the difference between 2x and 2.5x is the difference between falling at 800 BPM and 1000 BPM.
As a side note, I played C800 and C700 for nearly all of the 6 years I spent playing stepmania. I played o2jam songs at an effective fall rate of between 600 and 750 (150 BPM songs at either 4x when I started, or 5x after I trained myself to read 5x) and there were people who could read 150 BPM songs at 8x. Most if not all of the players who will be legitimately good at osu!mania are going to use very high scroll speeds (especially anyone who comes from one of the other communities).
This system still does nothing to address the problem of not being able to use 1 speed for all songs, unless you are able to read at a wide range of fall speeds. There are 2 ways to solve that problem: automate speed selection by specifying a target final fall speed in BPM and have osu! select the appropriate speed multiplier (this would also become more complex with maps that have a large speed range) or provide C speeds. Neither of these are likely to be implemented, making this something that you cannot solve with the current attitude that many members of the osu! community seem to have.
I also do not see why slider speed or approach rate are necessary in the first place. Playing osu standard maps in osu!mania is not the intended way to play osu!mania, and these options can be easily removed from the options available to the mapper when mapping specifically for osu!mania. I see no reason to try to make osu!mania into "standard osu! played scrolling downwards" it should evolve into it's own mode without being too attached to how osu! standard works. The mode is obviously inspired by Beatmania, O2jam, DJMax, Stepmania and other scrolling games, each of which had different options available. Personally, I preferred Stepmania and o2jam. Stepmania, because I could play C speeds, and a large portion of the community did too, and the maps were always extremely well timed and extremely accurate to the song, and O2jam because I enjoyed the 7k playstyle (but I hated how limited the speed modifiers were).
EDIT: also, if C mods were allowed, but were unranked, I would be extremely happy. I don't care about my rank, I play the game because I want to have fun. For me, fun is self-improvement. I will always be mediocre, so the things I need to improve will always be things like reading more and more complex patterns, or dealing with more complex holds, learning to play lifts, etc. I don't need speed changes and other things outside of the patterns themselves causing problems for me. I just want to be able to play the game my way, and if that means sacrificing the extra bonuses of having my scores ranked online, I'm completely ok with that.