In PnP, I totally agree without a shadow of doubt, but in a persistent world where we have around 35 players on at one time, probably at least three times that in active players, and with most players having more than one active character, the PC population reaches into hundreds, and therefore does become a significant portion of the world. Even if only 25% of all those 300+ active character are magically able, that's still 60+ casters. Forgotten Realms tends to have less than that in a city.
The PC's are everything from adventurers to guards to leatherworkers to preachers to beggars. They (we) ARE part of the world, and that isn't really a bad thing for a PW to have achieved. But it does mean we need to be factored into what the setting is.
And I know for a fact several NPC Barovians are magicians of some kind, judging by one who will remain unnamed summoning a Dire Wolf when a wererat chased me into a building!
Disclaimer: All the numbers here are made up, and hence are the third kind of lie.
Still, the point sort-of holds, even with less numbers. Daggerford, a city on the Sword Coast of comparable size to Vallaki, likely has about five able casters (Of level 10+, usually head priests, castle mages, and so on), and about ten or twenty less able ones acting under the higher level ones. Even if your party of PnP adventurers contains 3 casters, you're still not tipping the balance massively staying there. But if your PW of hundreds of adventurers was centered there, and there was suddenly 100 more Clerics/Wizards of varying levels, it would be like being dropped into Thay's annual mage-party.
Not suggesting anything outlandish like applications for casters, or massive restrictions, just encouragement for other classes based on the fact that a good chunk of folks, admit it or not (and myself somewhat included) like to have powerful characters. I bet if the tables were turned and Fighters were soloing dungeons Clerics couldn't do until five levels after them, the CC would be swamped with applications for Weaponmaster.
Veering off my own topic now. Bleh.