If a single 13th level druid can destroy any single non-magic using kingdom, why are there magic-hating kingdoms left in this world?
Wouldn't they have been destroyed under the stone hooves of Stone Destrider* riding armies wearing full plate, tower sheilds using, lances, longbows and longswords that a wizard could easily create from nothing using just Iron Wall + Fabricate?
*[The Stone Destriders being created by Bound Efreeti, a level 8 wizard can bully an Efreeti into giving him Wishes, and an 11th level one can just have a room of Bound Efreeti at her disposal, giving them as many as three Stone Warhorses per bound Fiend]
Wouldn't nearly every single Cleric or Druid just level the weakling non-casting cities for not allowing their dieties to be worshipped, or for encroaching of these cities on the wilderness? I'm pretty sure that even Pelor and Heironious would want to spread the word of their majesty; and the Elves and Dwarves and Orcs would all worship their Racial gods that created them. Which, funny enough puts humans in a massive disadvantage, as they have no magic, while their competitor species have full-casters with deific backing.
It really doesn't take that long for a character to reach level 20 (about 85-86 days with 3 encounters per day, minimum, that's just under 3 months; if you take a year, you are almost guaranteed to hit 20). Then again, you really don't even need to be level 20 in order to shatter a city that doesn't use magic, when you're a full spell caster. I think that a level 13 Cleric or Druid could probably do irreperable harm to even a metropolis (100,000+ people) sized city. Spells like Airwalk, Control Weather, Lighting Storm and Firestorm are more than enough to reduce a city that is even 6 miles across into a flooded bog, or a territory that is so dry that a few Firestorms or even Flamestrikes would light the whole place up.
Remember, unless there is a spell caster with the incentive to defend a place; most cities will seriously be destroyed once a single high level spell caster is offended enough to have to deal with a city that they do not like.
Also, D&D has powerscaling that rivals every other game system, once you are powerful, nothing below a certain power level even comes close to threatening you. At level 1 you fight Orcs, and they can seriously kill a Barbarian with 18 or 20 Constitution in 6 seconds with their axes if they get a critical hit. At level 8 the Bard can fight 50 Orcs on his own. Other CR 8 challenges are still going to present a challenge to him though.
However, at level 16; everything level 8 is just as trivial for a level 16 creature, as the level 1 Orcs were to a level 8 character. That's the sort of powerscaling that occurs in this system.
Just some rather interesting considerations to take into account. The high magic setting of D&D is sort of a circle. The powerful monsters in the game usually need wizard, cleric, druid, optimized rogue, or sniper ranger levels of character effectiveness to defeat; the result is that pretty much anything that's a spell caster in the core books are the only real classes that players should play. One half is the fact that only magic can stand up to the monsters in D&D, and the other half is that Magic users are the only ones that have the magic to deal with said monster.*
The D&D mechanics and game usually assumes that players will also have a magical equipment just to be able to "not die instantly" against challenges above level 4 and 5.
How are you going to address those issues? I'm asking since in my own DMing experience the only real ways to solve this are to simply give the players a very high array of stats, and use less magical items; or use the elite array (15, 14, 13, 12, 10, 8); and then use the assumed treasure rewards that each fight should give in order for player characters to be equipped to play the game.
*:This is assuming the core system + every splatbook published. Myself, and many of my gaming friends, use a series of alternate classes, equipment rules, spell rules, combat re-writes and more accurate "fluff" descriptions based on what a race or class can actually do in-game.
Like how... Sahaugin are seriously the biggest threat on any planet that they live on, but are fortunately limited to just the oceans. They're smarter, stronger, tougher and more dangerous than every other aquatic race (and more than humans as well), and have probably driven their rivals to the fringes. While say... Sea Elves are actually stupider than humans; making Sahaugin view Sea Elves intellectually the same way that a Human would view a Griffon's level of thinking.
Sea Elves, on the other hand, live in their equivalent of a post-apocalyptic world where monsters that are smarter, stronger and tougher than themselves prey on them for entertainment. The only reason that Sea Elves survive is that Malenti (Sahaugin that look like Sea Elves, so they're intelligent and tough, but don't have Sahaugin natural weapons like claws) commonly are the leaders of Sea Elf communities.
This content also has major game fixes, like the problematic spells like Wish, Ploymorph, and the Calling/Summoning spells. As well as having a Fighter class that is straightforward, can hold it's own against a Balor at level 20, and is worth going all 20 levels in. Knight, Barbarian, Samurai, Bard, Assasin, and Monk were all rewritten to be classes that you would want to take all 10 (knight) or 20 (the other classes) levels in.