Accuracy never becomes useless unless your opponent has exactly 0 defense(and as far as i'm aware, no monster in rs does), you just have diminishing returns. Given that the best combat xp is gained now by fighting higher level monsters, the accuracy will almost certainly help.
You're splitting accuracy from chance to hit monsters? Isn't that the very definition?
Accuracy is one issue, but the chance to hit monsters is a completely separate one, it's no use have more accuracy if your chance to hit your desired monsters doesn't improve (IE is capped) by using a higher tier weapon.
Give me an example of where accuracy is capped please - I'm actually unaware of any thing of the sort.
I am surprised that you are unaware of an accuracy cap...
The Chance to hit any monster is 100% (or whatever the game designer have put in, 99%, 95%, but the exact value does not matter in this discussion), regardless of how high your 'accuracy' is, this is true in every single thing under the sun, you cannot get more than 100% chance to hit.
While accuracy is very closely linked to chance to hit, it is not a direct conversion (if it was, it would mean that low level players cannot hit anything with bronze weapons due to their accuracy being so low). Thus there must exist a point in that calculation where the resulting chance to hit on the monster has reached the cap (whether that is the absolute cap or the game cap), and any further accuracy increases beyond this point serve absolutely no purpose.
Accuracy is a great thing to have, but once you hit that cap, it becomes the most useless stat to upgrade thereafter.
I am pretty sure that chance to hit bosses will not hit the cap anytime soon without using the very best gear, the rest of RS is a completely different matter. If one can reliably hit stuff without missing their targets for any length of time using a lower tier magical weapon, then I find absolutely no point in getting a degradeable upgrade that yields no actual upgrade. Magic is unique in this regard that the weapon only provides accuracy and attack speed, while the damage itself is determined by the spell alone, which has no tie to the weapon. Melee and ranged upgrades will always be upgrades because their damage is based on themselves rather than something outside, so even if accuracy is redundant, their damage is unaffected.
The thing to consider here is rather that a one handed magic weapon will be faster, thus you will be able to land more auto-attacks. With slow weapons like 2h's and the chaotic staff, most of your damage will be done by abilities alone, thus it would be advised to use a slightly less accurate 1h staff and a book when your opponent either has low hp or low defense(~>80% accuracy), or both. That and as ascribed above about the damage part is why magic is quite different from melee and ranged in EoC(and, I might add, has a distinct advantage).
I think the penance monsters in BA have 0 defense when using the correct attack style >_>