Going to bring a bit of reason to this thread so people reading this will not have an annurism.
I understand that, those who wish to have this specific dungeon level capped, are looking for a unique and challenging experiences. The R30 dungeon would be more likely to offer this if all party members are the appropriate level. The actual degree of difficulty is unknown at this point, but it's safe to say 4 level 30s will have greater difficulty than any combination of higher rank classes. Capping the rank, therefore, would then keep the difficulty roughly the same.
First, I would like to say, there is not much preventing any player from playing this dungeon only as a level 30 class. The obvious reason others want to cap the dungeon for everyone is to control other players' playstyles. Not necessarily for nefarious reasons, but it is true if the degree of difficulty is not the same for everyone, there is a lesser sense of accomplishment by arbitrarily choosing to engage an encounter handicapped. But the fact is, it is more to prevent other players from doing something than from wanting to do something your own way, as noone is stopping you from collecting a group of r30 players and trying the dungeon. However, there are limitations; if experience is offered in this dungeon, eventually all your classes will outrank the dungeon and you will never be able to attempt the dungeon the way it was intended. Although it is possible to level multiple characters, it is impractical to expect players to constantly level characters and classes to do so. It IS unknown whether experience is offered at the moment, so it would be pointless to speculate any further to this point.
Now for players wanting to NOT have this dungeon be capped. Obviously every player has their own preferences, so it goes without saying that some or many players will want to 'steam-roll' through this dungeon. It is their preference, and it hurts noone outside of those who agree to group with them and those who feel their experience lessen others' own achievements. Not much more could be said to this point.
That said, this dungeon is intended to be a stepping stone. I don't believe it was intended to be seen as endgame content or some kind of dungeon for players to continually experience. More likely it will offer decent experience for r25-35 players and the chance to acquire gear to advance them into the higher levels. In short, it is an alternative to leves. The required 4 players for this dungeon make it optimal for a party of 4 to all be the appropriate level, as to maximize their SP.
In other words, the dungeon will be incentivized in a way that makes players want to be r25-35. However, if they choose to have a r50 in the group, they gain nothing other than a quicker run potentially at the cost of some SP. My point is, if you want a sustained challenge, look foward to the r50 dungeon, this r30 dungeon is intended to be something else entirely. ALSO consider the delays that would have been caused by implementing the code to restrict skills of a certain level and reduce the caps of characters. I think when you consider all these points and you still want a cap for the NON END GAME 'stepping stone' r30 dungeon for it's 'challenge' (that will already be diminished as its balanced for brand new r30 characters), and still wish for it to be capped; then you are most likely to biased in this particular situation to see clearly.