the "plot power" point assumes perfect omniscience on the part of a wizard player who needs to balance int mod+wizard level prepared spells against spells they manage to collect in their spellbook and every possible situation they might come into contact with. Back in the 3.5 days when you had
vancian magic it meant that level x y & maybe z had a niche spell that could really save the day where the marginal cost to pull that ace out of their sleeve was to devote one of the possible 1-4 slots for that spell level to that spell
I played from OD&D on, so yeah, I know Vancian

5e's casting is a lot more versatile. You can bend spells to different slots, you can always use all your slots no matter which spells you have 'memorized', plus you have rituals, which obviates even memorizing stuff like Detect Magic (you might do so anyway, depending on your needs, but not doing so doesn't make it unusable).
But the real thing is, plot power isn't about what you can whip out at the spur of the moment when trundling through the dungeon. It is more about how when you're all sitting back in the tavern figuring out how to beat the dragon, the key is ALMOST always some spell trick or other, and often there will be 2-3 real enabler spells. Of course those are going to be memorized, or employed before engagement, or committed to a scroll! Back in 1e days "Questioner of All Things" would do exactly that, just memorize line of battle spells, and put everything else on scrolls. Always had that wacky oddball spell. Items also helped a lot there, as they pretty much became the go-to for basic combat 'casting'. 5e DOES tone it down a bit, but not that much. The wizard is still boss man if he wants to play smart.
Warlock still has huge spell lit overlap with the wizard to further erode the "plot power" argument, but w hen they changed how spells were prepared so preparing a niche ace in the hole spell has the same opportunity cost as a daily driver bread & butter spell they can expect to depend on most sessions or long rest to long rest. It doesn't help matters that they over used concentration, energy resist, lowballed damage & all sorts of stuff to thwart linear fighter quadratic wizard in a no feats no magic items game & wound up inverting it in a normal game with both while the wizard/sorcerer/etc is still left with a quiver of spells that are generally almost good enough to just keep pace at their best.
Well, I wouldn't strictly limit the plot power to just wizard. It is a trait of full casters in general. Though wizards are the typical examplar of it. Clerics and Druids have a share in this act as well! I'm less familiar with the Warlock and Sorcerer in play, they seem like their spell selections are a bit more 'tactical', but I'm sure it also applies there to an extent.
I did NOT find, in the game I played a Transmuter Dwarf, that I was just keeping up with the fighters! Not at all! I'd say I was feeling fully their equal in combat, and outside combat I was pretty boss. I do tend to have a mind for exploiting my wizards, but I also think they benefit a LOT from magic items. Anything that will let you free up slots for those edge cases and non-combat situations is gold.
Yes they can start with it, but the fighter chooses from "chain mail or (b) leather, longbow, and 20 arrows" & is more likely to choose the chain mail while a paladin can choose from "
a martial weapon and a shield or (b) two martial weapons" & is again not likely to choose a long bow because they aren't really a ranged class & the javelins are probably good enough to help the raged types until things are close to melee if it comes up. If they find that a longbow is needed so often that they really need to invest in dex & longbow combat thy can do that later but probably don't at level 1. Your 1st level spotlight example wasn't just an niche abstraction it was an isolated white room example so far removed from actual play as to be irrelevant. Concern about the spotlight at level 1 is so minimal even in a game that drags things out like my precovid campaign where the party spent about three months of weekly games just surviving & gathering basics before reaching first level. I believe they were 4-5 within a couple levels of hitting first. Wotc can't balance casters other than warlock against the assumption of perfect omniscience 100% of the time both in the adventure the gm plans as well as against other players... They certainly can't do it assuming that the resulting just keeping even at peak vrs at will with normal gameplay where magic items & feats are a thing so it works with no feats no magic items if their HCs rain down magic items & new books keep releasing more feats as they have.
Eh, starting with the leather option is a perfectly valid choice. That is exactly what my fighter did! I just built a primarily DEX based fighter. Heck, he's a melee type to boot, but even so chain wouldn't have increased his AC any to start with, and he can get good effect with the bow, and uses it on a regular basis. I have no plan to stop using light armor, it works fine. My AC isn't the highest in the group, but it is only a tad behind the paladin, and I can dish out a real lot of damage in almost any situation, whereas she's really only good in melee and REALLY only top tier against undead (we have met a lot of those, so she's pretty happy).