MuhVerisimilitude
Adventurer
Every time you make something a spell you make it inherently less unique, and it becomes just a part of a spell list. This makes the class using this less "specific", since now the ability is optional for this class.Too far with spells? If the ability works well as being cast a flexible numer of times, as with spell slots, or as a cantrip, and would benefit from the spell structure (borrowing all the rules of the spell system like action, duration, effect, whether it counts as magic/a spell for certain types of abilities), then why not create it as a spell, and lean on that existing system? The design has to be written anyway. You either place it in the spell section with similar designs, or you add the ability, plus all relevant side rules that answer the relevant questions into the class write-up, padding the page count of that section.
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As a comparison, I'd rather have a class ability or monster stat block reference a spell to look up, rather than write up that entire spell in the class or monster stat block.
This is why Eldritch Blast should obviously be a spell, because it shouldn't be optional. It's like a fighter: Weapon attacks for a fighter are not optional. Eldritch Blast is what the Warlock is designed for.
Sure you could argue that this flexibility is "good", but just go all the way then and consider what happens: If every class feature becomes a spell, then why even have a class system? The purpose of classes is to have some kind of archetype framework. If every feature is a spell, then every feature is optional and unless each spell is unique to each class then every class with a shared spell list is essentially the same class.
Secondly, and this is my even bigger objection: If you make everything into a spell then you are even further reinforcing the absurd martial caster disparity. Because now you have constructed a strictly magical spell-casting system that is exclusively for spell casters and then you have no similar system for non-casters. You have granted even more unnecessary flexibility to casters and you even allow them to opt out of trashy class features. I mean this system would never allow a fighter to opt-out of Indomitable, would it?