Short answer: There should be an extensive range of difficulty options for every player, which means that there should be an extensive range of ultra-difficult songs available that DDR currently lacks. Every song should have an oni chart of at least 8 feet, and if a song's oni is too easy then make an additional "groove radar special" to fix that. The distribution of difficulty should be naturally determined by how hard the charts are and song difficulty should not be planned out beforehand. The maximum difficulty range should equal or surpass 15-footers and the maximum difficulty of each subsequent release should equal or surpass that of the prior release.
Long answer: There was a very large discussion about this at http://www.ddronlinecommunity.com/forums/3/forum_topics/54 which posters should review for a better grasp of the points that have already been made on the issue.
Since I don't feel like paraphrasing I'll just repost:
"There's not a good argument against having difficult songs. The timing on DDR is already extremely hard compared to more popular music games such as Guitar Hero, and timing affects every player, while hard songs only affect those that somehow manage to venture to that difficulty. Yes, IIDX's timing is harder, but Stepmania on judge 7 is harder than IIDX and that doesn't make DDR's timing any less difficult comparatively. Arguments such as "well, if you put hard songs on the game, it will ____" are really just guesses because conclusions such as those have no R&D/statistical data to back them up.
Additionally, DDR is being promoted as a fitness tool across hundreds of gyms nationwide, and it currently fails at that goal. I have mild asthma (I don't need an inhaler, but my lungs feel like they're crushed if I exercise too hard) and I've gotten to the point where 10 footers don't exhaust me anymore. In my opinion, DDR is a very efficient exercise tool because you know exactly how many steps are in a given song, whereas if you run or do any other form of exercise, the amount of exertion is more dependent on how you're feeling at the time. DDR not only places a psychological demand on the player to finish the song, but it also requires a given amount of exertion.
Part of the reason they probably don't have many harder songs is because 1) they follow a pyramid structure, 2) each boss song is really not that hard, so they have to force it to be hard with gimmicks and excess arrows.
Regarding 1), the "pyramid structure" is opposed to "natural distribution"--with a pyramid, you have one really hard song at the top and the easier songs get more common as you go lower on the pyramid. This is extremely inefficient and unnecessarily grandiose, because with natural distribution the charts of a given song are just stepped as appropriately as possible and the difficulty is distributed accordingly. The end result tends to be an equal distribution of difficulty--that is, 10% 8-footers, 10% 9-footers, 10% 10-footers, etc.
Regarding 2), Dead End would be one of the hardest songs on DDR if they used the full potential of it, but it's a 9, while songs that are comparatively easy such as Pluto Relinquish or Legend of Max are forced into the 10-footer mold just because they sound dramatic. The people that make these "boss songs" aren't exactly changing their musical style very much, so as they continue to make music and the stepchart makers continue to best each past "boss song" the songs have to get more gimmicky with more excess arrows.
Finally, I don't see anything based on BPM or note reading as a legitimate form of difficulty. At best I think it's a distraction, because it's just an intellectual exercise, and at worse I think it worsens the game by putting focus on elements it shouldn't put focus on. The reason for this is because it's entirely mental and does not translate into anything physically--you can have bag display at 60bpm or 600bpm, but regardless your body would be doing the exact same movements. Every song should really be on C400 or C500, such as in this video where there are obviously BPM changes yet the scroll rate remains the same. I think this anti-physical mentality from the higher ups is a problem that hampers the game's development."
(I encourage you to view the original thread because virtually every possible critique of this post has been made there.)