As far as I can tell there are a few fundamental disconnects going on here that are clouding the issue of spellcaster breadth of conflict resolution power (and their corresponding ability to impose their will upon the narrative relative to their mundane counterparts) either being a non-issue or being able to be resolved GM-side. Some of all of these maybe carried by various participants.
1 - There is an implicit setting assumption of pervasive magic such that barkeeps, merchants, lords, and scoundrels alike all have a healthy dose of exposure to, and understanding of, magic. There appears to be an assumption that spellcasting PCs are in no way extraordinary, that all Kings and Councils will have a Court Mage, that all monstrous adversaries will have tangled with, survived, and learned to identify and strategically/tactically deal with spellcasting opponents.
2 - There is an assumption that GMs should have internalized the reality that their game should consist of an arms race or a rock/paper/scissors match with spellcasters (and somehow this ironically doesn't reveal that, if you must do this, and clearly you must as most modules make special note of their efforts to circumvent spellcaster plot derailment and encounter trivializing, that casters do in fact have more breadth, scope and potency of scene (re)framing). Further, there is an assumption that GMs who do not wish to adopt this adversarial approach to spellcasters have no experience with this type of GMing.
With respect to 1, I see a great many responses and interchanges just presupposing this and then subsequent incredulity when folks don't work from this presupposition and accept responses based off of this premise. (A) In my games, my spellcaster PCs are special. Magic is not so pervasive as to be common enough that most anyone would understand the myriad mysticism of a Charm spell. If the rules are agnostic on things, then I am not going to adopt an adversarial position, change my implied setting and suddenly start assuming that Court Mages are ever-accompanying all council members, that Bards are always traveling with merchants to chronicle their financial endeavors, that all innkeeps "know a thing or two about a thing or two (including magic)", etc. (B) Even if I accept the premise that magic is pervasive and PC spellcasters are not unique, I fundamentally do not accept that exposure to magic would make layfolk, and primitive monsters, and bourgeoise suddenly learned in the art such that they can reliably be expected to identify magical gestures, incantations and specific spells especially. We live in the information age, a vast technocracy where all that you could want to know is a point and click or an easy library excursion away. All of this available information and exposure to phenomena and your average person is utterly unlearned about their home plumbing infrastructure, the pitch of their roof, insurance policy provisions and exclusions (specifically homeowners and medical), how to change oil/brakes/air filter on their vehicle (all outrageously simple and considerably cost effective), how to cook more than a few dishes (if that), the proper maintenance schedule of their HVAC system, how to properly budget and account, and god help you if you ask them whether the Soviet Union or Italy was allied with the Allied Powers of WWII. These are ABSOLUTELY FUNDAMENTAL to your average person's existence. If this is true in the modern era (with all of the infrastructure and exposure in place to make it not so), then why in the world would we presuppose that layfolk, primitive monsters, and bourgeoise are arcane adepts (or at least functional). The only reason I could assume that is a gamist/pawn stance agenda that needs it in play to manage an arms race versus, and a rock/paper/scissors game with, PC spellcasters who predominate conflict resolution from mid to high level onward.
With respect to 2, I know myself personally (and assuredly many/most others) are extremely adept with, and considerably exposed to, classic Gygaxian Pawn Stance, Gamist 1e Dungeon Crawl GMing. I know every single trick in the book to challenge or outright "handle" spellcasters. And I also know just how many of the power plays expected in tournament play were utterly, utterly contingent upon having the correct spell in play (which is why groups generally defaulted to 6-8 PCs so spell-resource breadth was properly covered). Its great fun for that style of game. However, I don't want that in play in my standard, long-running campaigns. I don't like the fiction it creates, I don't like the muddled, genre hijinx that burgeons forth from it, I don't like trying to massage that genre with GM force after its gone pear-shaped (due to the clown shoes and spinnig bowties of a non-stop cavalcade of 10 ft pole prodding, ear monsters to stop listening at doors, drowning dungeons with decanters and jumping into haversacks for infiltration or damage avoidance), I don't like juggling the mental overhead that comes from the incoherent coupling of Gygaxian adversarial GMing with fiction-first, genre-logic GMing, and I don't like the table dynamic it induces twixt PC and GM. I'll take one or the other but not both simultaneously. I don't want haggling or table handling time over fiddly resource accounting (spell components) and I'm of the same mind with respect to serial accounting for granular units of time (with respect to spell durations) and GM-forced, contrived metagaming (not fiction-first, genre-logic...flat out gamist metagaming) of pressures to consistently prevent 15 MWD scenarios.
Beyond that, I see answers in this thread to common spellcaster issues that aren't just contrived, spellcaster-specifically, metagamed, adversarial approaches but they honestly just don't work out in play. "Why, if any of my Wizard PCs ever dared to consider flying off and scouting out the mountain by themself (while the Fighter et al climbs...slowly...and exposed), well they would learn their lesson the hard way as here comes a random encounter of wyverns (or whatever) to eat their lunch!!!..." Yeah, except if he's flying over the mountain to scout it out, he's assuredly invisible (possibly improved invis) and has something like hold monster/slow or some other SoS to end an encounter and potentially have his flying, bestial adversaries (which wouldn't have been able to see him in the first place) falling to their death after their failed (extremely poor) Will save/defense is bypassed. Then I see assumptions of heavily fortifying mundanes with large amounts of magic items (what absolutely must be vastly askew of WBL guidelines in order to get the derived numbers) while spellcasters having access to even a few of their items du jour (maybe a few scrolls and a wand or two...of which they can make and have PC build resources committed to making them so the GM doesn't even need to award them) is scoffed at. And then I see a proposed ruling of which if turned on its head would cause enormous problems at a table if deployed by a PC versus monsters; imagine a Druid PC casting a 6th level control spell on a pack of Giants and the Giants jumping on each others shoulders to cheesily, metagamily bypass the RAW and spirit of the spell (both the damage and movement portion...not even just one of the two control elements...both) via exploitation of encumbrance rules interfacing with the RAW loophole in the text of the 6th level control spell.
There are so many fundamental disconnects to get past that it seems pointless to even attempt to further converse on these issues. I expect our games wouldn't look terribly unfamiliar to each another but our table agendas and resultant expectations are worlds apart.