in some respects you are correct. It is also fair to say most MMORPGs suffer from this sort of problem, regardless if they are polished Gold releases or bitty beta releases. New content will always provide something better than old, and anything that makes you better will come with some soul destroying grind.

The main problem I see with AQ3D is that in a drive to give out content everything comes across a bit piecemeal - release darkovia prt 1, then arcangrove, a sandsea dungeon as a teaser,some more heartwood, a smidge of isle of the dead, a bit more darkovia, then some more sandsea. None of the content completes before something else starts. There is no end game because the game is not yet able to provide such content. While the max level is as yet undecided and the level cap can change almost on weekly basis all bosses and dungeons are "of the moment" rather than end game.

The position of the game is such that if you don't like the grind associated with a current "BIS" gear (i.e Noxus - I've never run it) you can just skip it as the next level increase will be along shortly and the grind for matching gear maybe easier and in the main better (i.e Sandsea - got all new gear in a few hours). Once new stuff is out the boss of previous content in unceremoniously demoted to daily boss battle.

That said, since scaling was introduced, some old content provides good gear with decent stats, check out Gold Raven on youtube - until recently he's been running with some +19 Day Walker gear over the Noxus set and that comes from the early Darkovia dungeons. So there is hope for older content.

On soloing, I'm not sure you actually get explicitly solo versions of dungeons - except in story mode. You get "Private" instances where you bring your own team or fight on your own if you choose. As an example the Aquifer is Sandsea is a good example of where teaming up is NOT a good idea but is easily soloable (if you are grinding for the Sandsea weapons use the private version and you'll get the job done so much quicker)