Quoting from another thread, rather than re-framing my proposal, since I feel it works well as such...
You're supposed to cycle out spells as new ones become available. In fact, the progression of spell choice is such that, initially, your spells at your highest level typically are reserved for being your go-to spells for dealing with monsters--nukes or heavy CC. Then, as you gain new, better spells for fulfilling those roles, the previous slots can be changed over to be dedicated to utility spells (buffs mostly) or secondary offensive spells used to diversify your repertoire. Additionally, individual spells don't define characters, so obsoleting some isn't all that problematic.
The same is not true of heroic abilities once you hit epic levels where they aren't replaced by upgrades, but instead simply made useless by no longer growing to match the increasing scale of the epic levels. Some of these abilities are central to a character's concept or role, so when they cease advancing, those characters are left feeling incomplete. A small list of those abilities include: animal companions (druid, artificer, Palemaster), monk finishing moves, Assassin poisons, Turn Undead, and numerous other class- and enhancement-granted abilities that rely on increasing DCs, or scaling benefits, to remain relevant, yet are not advanced by epic levels. This is a major problem with the current epic level/epic destiny system, and something that was pointed out long before MotU went live.
A possible solution would be to change standard epic level advancement: at each epic level, characters are given a choice from among any of the classes in which they have levels. That epic level is considered to be a level of the selected class for the purposes of advancing class features and class-level-based enhancements. This would include caster level. On top of that, features that have a non-standard progression, such as animal companions, should gain epic-level advancement that comes along with the chosen epic levels.
Each class should have a few new things coded up for their advancement. At an absolute minimum we could have things like:
Wizard/sorcerer/cleric/druid/favored soul/artificer/bard/paladin/cleric - +1 caster level. This makes more sense here than in epic destinies, given the design of the ED system to both almost force players into spending a lot of time playing in destinies not associated with their heroic classes/roles, and the trend for many characters to end up in destinies that do not sync with their heroic classes (many arcane casters are in Shiradi, most paladins are in Legendary Dreadnought or Fury of the Wild, etc...).
Paladin - +1 to paladin level when determining formula for Smite Evil damage and healing from Lay On Hands. Continue progression of Smite Evil uses/rest.
Druid/Artificer/Palemaster - +1 to level for determining benefits of animal companion (increase HD, HP, attack bonus, saves, damage, and include some epic-level abilities to keep them from being absolutely worthless post-20).
Rogue - +1 to rogue level for determining DCs for all rogue abilities (poison DCs, Assassinate DC, trap DCs, etc...).
Monk - +1 to monk level for determining DCs for all monk abilities (Stunning Fist, Quivering Palm, finishing moves, etc...). This one probably also means having to scale back some of the benefits afforded SF and QP made to bring these up to being useful for endgame, either turning off Shatter's bonus applying to QP, or having it only apply at 50%.
Barbarian - continue progression of Rage uses/rest.