So, I'm wondering, if a fair number of people have consistent access to 9th level magic, what sort of implications would that have for how "adventurer society" acts?
Assuming that spells of 3rd level and below are "common" as well as 20th level characters being, say, 0.1% of the population, I would envision significant vertical urban centers. Many of the taller buildings would be constructed as exotic as possible to demonstrate strength, durability, utility, and aesthetics. In significant capitals and trade centers would have the most dramatic designs (
c.f. Hong Kong architecture competition). These capitals would also have elite access teleport centers. The teleport grid would be heavily utilized for anyone of significance. You might need to have some quality marking you as 9th level or higher, but I can see there being a key, ring, or some other item that allows access even for non-spellcasters. I can see this becoming similar in commonality as air travel.
If
purify food and drink doesn't allow for desalination, surely a bound water elemental would suffice. A 12th level caster can cast
planar binding for a month's duration. That would be sufficient for the crew's water needs, particularly with another water or air elemental providing propulsion. This could easily be as quick as second generation steamships, taking less than two weeks to cross the Atlantic Ocean. Bind a lillend or a djinn and you have a cruise ship with food and entertainment.
Rural areas would be more expansive and heavily managed. As mentioned earlier, currently available cantrips and 1st level magician spells replicate tractors efficiently. Tamed ankhegs are living plows, and orchards can be easily managed to high productivity with persuaded dryads and/or treants. Livestock can be similarly managed with fauns. The hauling of produce, ore, or refined materials I can see by golem hauled wagons, reinforced with steel to allow even greater amounts of cargo. If the teleport network is air travel, this would be rail. Slower, but quite steady.
You're going to need that extra food because your infant mortality rate will plummet. Between
cure wounds and
lesser restoration, 1st and 2nd level spells now, childhood maladies and accidents can be easily managed. Even working accidents among adults should be manageable as long as they aren't short-term fatal. Woodsman Eugene need not be as careful with his axe. Only magical plagues would be of concern. The first appearance of the Red Death would be terrifying as lower level clerics were powerless, but it would be contained in a straight-forward manner when the hierophants and archmages became involved.
Monstrous issues would be relatively contained outside of political borders. The lords of the nation (fighter, rangers,
et al.) Would at least push such creatures to roam outside the borders. This could lead to strategic wilds; geopolitical "border states" where the fiercest beats roam. Agile fighters and rogues perform spycraft to discover tactical and strategic intelligence, with the occasional assassination of prominent merchant, noble, or sage.
Magic items would only be linearly more prevalent, not geometric or exponential. All of the main function of spells is based on duration, even those that rely on summoned assistance. The enchantment of magic items is a craft, and can not be industrialized. It still takes a living mind to create a +1 sword. This is not something that can be generated
en masse; you can figure out how to make lace through water mills and automated looms, but the binding of magic can't be so generalized.
Wish is not as amazing as it might seem at first. While the price was rough in earlier editions (5 years of life), now the cost to
wish is meaningless. Unless, of course, you are doing something other than mimicking a spell. While there is still a relatively minimal cost, odds are any particular magician is only going to be able to be inventive with the spell twice in their lifetime. While I generally dislike to consider personalities in these thought experiments, here I believe it matters. I find it difficult to imagine any 20th level character, but most especially a wizard, sacrificing great power for society, or even mere wealth. I can only imagine that it will be saved to the benefit of themselves or their friends and family.
I think there would be societal advancement along an Industrial Revolution track, but limited. The greatest of innovators are at the very top, and not facilitating further advancement among their lessers. Usually, I argue against the stifling of technological advancement with strong magical advancement because of the craftsman requirements that magic imposes. The peasantry aren't going to be able to call on their own earth elemental, so they will need to (eventually) discover principles and inventions that allow them to achieve the same goals. Magic becomes the tool of the elites, and technology the tool of the masses. Here, however, if magic is common at all strata, and the highest level are very high indeed, I think you end up with a Victorian England society in the vein of The
Amulet of Samarkand, by Jonathan Stroud. In these stories, the elites are the magicians whose only power is the binding of "demons". Their magic items are tools that have these entities bound within them, called upon to exercise their powers on demand.