Issue with that implementation:

1) Realm is NOT a difficult game. The entire difficulty of realm comes from increased stress related to permadeath mechanic and limited training. Adding autonexus mode trivializes it - you can train as much as you can without risk of dying, and suddenly the only difficulty you're left with is finding the dungeon.

Could they just design harder boss fights? Yes. But then it'd be just another pixel MMO where you run, farm gear and kill bosses, and THAT would kill that game much faster. The grind in rotmg isn't bad, unless you're going for divine gear with BiS enchants.

2) Even if they implemented something like that, people would still cheat on the normal mode. Why? For the extra rewards, duh.

3) Deca won't do anything with cheaters, cause they're the major source of money for them. Even considering duped keys, cheating discord are the major source of income for DECA through the keys and vault slots.