Defence and attack don't really plateau though
They do.
I suggest you look through this users’ posts. Swan reverse engineered most of the formulas in this game.
memberlist.php?mode=viewprofile&u=6295
Have fun looking through the data, it should have all the answers you seek.
My face when people bring up Swan's formulas
While they were helpful... when max stats were 1500 and max abilities 4000, Swan's formula's just don't work anymore unfortunately and need to be revamped. Players like Krummy have started the process but it is no where near complete (we have new mage formulas, a new auto attack formula, and he is currently working on new druid formulas with DemDruidsDoe)
As for whether or not attack/defence plateaus, in short yes, in long answer, no.
By the standard game definition, an attribute "plateaus" when it reaches a point where adding more increases the desired secondary attribute by a negligible margin, usually denoted by where investment in another attribute starts to outdo investment in that attribute.
If you put a toon in a vacuum, ie you do the math for a toon outside of boss evasions etc, there is no attack or defense plateau. And from a continued game standpoint, it makes sense for players to act like there is no plateau as otherwise when the next boss comes out they are behind and not ahead.
In a more practical sense in terms of current gear prioritization, your defense and attack attributes do plateau on bosses at around 15k usually, with 10k being the normal sufficient amount.
Because MMO's are a genre of continuous releases and continuations, I don't see a reason to mark a plateau on attributes that don't play into strict formulas. For example, fire magic and focus plateau for a fire mage because there is a strict point where more points in these does negligible work for fire bolt and other fire mage skills due to the formula for fire bolt. Also as energy is pretty negligible for an EG mage and fire magic doesn't really work for accuracy measures, it is easy to say these attributes plateau. Something like focus and attack are boss dependent though, and thus it is up to you as a player to assign a value on how much you need. If your goal is to set yourself up for the long haul, then there really isn't a plateau, but if your goal is to make yourself optimized for the current game play, then the plateau is whatever the max attack/defense of the current EG boss is, in this case BT and Gele.
I hope this helps clear up some of the confusion
As always, if you have any more mathy related questions I'd be happy to answer
--Bob