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
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.
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.