Well, this is kind of a bad argument, because melee is then only thing that you can expect to do at a low level. Archery is damn useless until you're level 25 or so and that's if you forego your ability to do anything else. Once someone else approaches you, you won't have enough power strike, melee wpf and athletics to be effective. Cavalry requirements just got boosted, and you won't be able to use a worthwhile horse and decent power strike until roughly that same level and then once dehorsed, you're a sitting duck for the same reasons. An even more specialized class like horse archer? Forget doing anything until 30.
Also the climb to 25 has recently been expedited. So, in reality, I don't think the balance team has any ambitions to balance the grind at this point and are focusing more on the reality that is 30 and up.
By "low" levels I was thinking of 30-31. Hence the quotation marks. The sub-30 grind is another discussion entirely.
Say you want a 7 riding Lancer/1H build. Currently, at level 30-31 you can go 15/21 and use every lance and 1H in the game with that. If the difficulty of the best 1Hs or lances was increased to 16+, that build would have to sacrifice key skills or the 7 riding to use the best weapons. That's not a problem in an isolated case, it's fine that someone has to sacrifice something to wield the best weapons. You could balance the game around that, no problem.
The problem is this: A level 35-36 Lancer/1H could have 7 riding and up to 21 strength without sacrificing key skills. That character could have the best horse and the best weapon, while the 30-31 character could not.
If the game was balanced around 7 riding characters
not being able to wield the best weapons, the high level build would be overpowered.
If the game was balanced around 7 riding characters being able to wield the best weapons, the low level build would be underpowered.
In other words, increasing the difficulty of the best weapons would reduce AGI stacking, but it would also make "low" level (~30) agi builds strictly worse relative to high level agi builds (on top of the inherent stat advantage).
TLDR; Balancing the game by increasing difficulty on items only makes sense if everyone is the same level.