The EN World kitten
Why is that implausible? Where is it defined that these 'deep reserves' must be generic?
It's implausible because it's not defined. You haven't told me what these "reserves" are, and so without anything to go on it's more plausible (via association with the fighter's concept as a non-magical character) to presume that they're physical abilities.
If that's not the case, then give them greater definition, because by itself "reserves" doesn't tell me anything. If it's not informing me as to what it is within the context of the game world then it's not associated.
Why does it matter that it's not magic? Are you trying to suggest that non-magical abilities are all generic and call upon some single, undifferentiated pool of potential?
Usually when one thing tires you out so much that you can't use it again, that level of fatigue affects your performance in other areas, yes.
Why not? Characters in fantasy are clearly capable of quite a range of remarkable feats, yet they don't just do the most powerful/effective one every time - who's to say there aren't some limitations on which they can do how often to explain why that is?
We are to say, because unlike passive audience members we're the ones who get to peak behind the curtain and see the "how" of things. We get to see what those limitations are, and can determine if they make sense within the context of the game world or not.
What kind of detail are looking for in defining 'exactly' what 'deep reserves' represent?
Starting with if they're natural or supernatural would help. "Magic" carries an inherent definition of "not bound by conventional understanding of things," and so in turn can set its own understanding for how it works. "Reserves," by contrast, carry no such inherent definition. Just throwing that term around doesn't tell me anything about how they work within the context of the game world, and so fail to associate.
Why can't they be discrete? You keep saying they aren't or can't be, but don't offer a reason.
See above. If you're not defining these as magical, then why should they be discrete?
neo-Vancian is the 5e system of separate prepared spells and 'slots' that are used to cast prepared spells (you no longer lose memory of the prepared spells). AFAIK, 5e has yet to offer a rationalization for the system, though I coiuld've missed it.
Magic doesn't need to rationalize itself; how magic works is inherently self-explanatory.
For that matter, what's so reasonable about classic Vancian - where spells are 'impressed upon the magic-user's brain' and 'whiped clean by casting,' - which isn't so much a reason as a re-statement of the memorization mechanic? What's plausible about the energy of AD&D Vancian magic coming from a positive plane that the material components of the spell are swapped to, when not all spells have components, and some with large components arguably involve more energy than those with less or none?
See above. Because magic has no inherently defining characteristics - beyond that it's not bound b the conventional understanding of the way things work - it gets to set its own rules.
Any or all of those are less reasonable or plausible than an heroic fantasy character calling upon deep reserves to perform a preternatural feat.
No, they're more reasonable. We know magic is inherently supernatural - we have no idea if that's true for "deep reserves" (though here you seem to be implying that they are, since you said that fighter abilities are "preternatural").