Out of the three broad categories of melee, ranged and caster, I don't think it's immediately clear that melee are greatly ahead of both. Ranged is still very powerful, particularly arti/inq.
It is abundantly obvious that many caster classes are behind ranged and melee. The r7+ nerf to caster damage was probably never needed, and is now a bit silly. Their burst DPS is behind and it's not as sustainable.
Should that nerf be removed? I would rather see that same curve applied to all, as r10 is indeed too easy in many cases. The only real challenge left is trying to push higher skulls in raids, or maybe some specific higher skull solos like the last one in TOEE or those Sharn mind flayer challenge quests.
They decided a while ago that they wanted DDO to be a solo game, on most classes. Casters got SLAs so they wouldn't run out of SP, melee and ranged got self healing, AOE damage and CC. Everyone gets everything, thus there is no real need to group at all in any situation when leveling, and even most situations at endgame. The issue is that melee and ranged just got more overall, and now there's just no need for casters at all.
DPS (and survivability) has gotten very high FOR THE BEST PLAYERS, so they have to have high HP mobs to make things not die in a matter of seconds, but for the average player this results in a boring slog. The gap between ultra-geared-uber-completionist-perfect-build-mythic-reaper-p*mped-out-character and someone who doesn't really know what they are doing is truly massive, that's a far more important issue than the overly simplistic question of caster vs melee vs ranged.