It's not the monsters' levels that are the problem

They don't lack for places to train, but they feel they could train faster if the formula would only let them challenge higher-level mobs. And the reason they feel that way is because mobs their own level are no challenge whatsoever.

The answer to the question is in the question itself. Ohnoes monsters my level are to weak and I can't train on higher level mobs cause they're stronger? Lolwut?
Nothing is stopping you from training on higher level mobs (xept maybe at higher level the Time Temple quests >_<). The misses make up for more hits/kill, making them more challenging.
The REAL problem here is that people freak out every time a "miss" pops up on their screen. Or people that only care about having the highest possible exp/hour at all costs, like everyone who spend all their time at gallos a week ago.

I've been training my BaM on mobs level +5 than mine. At least they make me use my finisher. I'm not sure if my exp/hour is as good as training on my level mobs, but at least this is abit more challenging and so more fun.

What if they instead of 5% missrate /level they would implement a -5% damage and +5% stun/pull/freeze/whatever resistance. The effect is exactly the same, but there would probably be less peaching. "fixing" this to make acc count for reducing this effect would greatly imbalance thieves/bowman, unless they remove acc bonus from dex and luk.

Accuracy has been completely replaced by damage. Say a person training on level x mobs tries out level x+10 mobs, so there is a missrate of 50%. If he doubles his damage, doesn't he get to train at x+10 level mobs now with the same ease as level x mobs? (Not counting stunning/freezing etc. difficulty).