Exactly. What's the problem? Loading up a spell-set for a specific fight is bad? Going to be fighting undead, well instead of clicking the 3 undead-based spells on your hotbar of 20 damage spells, you can load up the ones appropriate for the content you're about to tackle.
How is having 6 'best' spells for a situation any less 'same as each other' than having all your spells available?
I'm okay with focusing on playing better. And having limited spells IS playing different. If you're the guy who loads the one spell that most wouldn't load in that situation and it saves the day, you're a hero. You used your situational awareness and knowledge to prepare for certain circumstances, and then execute the plan you thought through in that circumstance. As opposed to just having all your abilities right there like everyone else and clicking the right spot on the UI, like anyone else. The former being a game of knowledge and awareness, the latter being a game of UI whack-a-mole.
I don't think you played or understand how EQ played. It wasn't like GW2 where you have the choice of weapon, then have the same limited tools as everyone else, which lead to a pre-determined rotation you stuck to, that everyone knew because you were carrying a certain weapon.
If anything, a limited loadout system like EQ actually talks about what you want by making there be even more room to differentiate yourself. As a Beastlord I used to load my heal spell that most only used solo. I had the heal boosting AA's. On quite a few occasions in raids or groups my spot heal of the MT saved the day. Holy shit, I'd planned for a worst case scenario, saw an opportunity, executed my plan, and everyone was like well done because most BSTs wouldn't have had that spell loaded or would have just used it for self healing.
There will always be a perceived optimal setup. With all spells and abilities available all you're doing is adding a ton of clutter to my screen and making it a game of find-the-hotkey instead of using knowledge and planning to get the right spells at the ready. Either way, bads will be bads, and everything you say could be said in defense of the system you're arguing against, which is why it doesn't make much sense to me.