Comparing adventurers to teachers isn't really a good example... Teaching, on a pre-college level, is a very necessary and therefore common occupation, and doesn't have a relatively high entrance requirement in the grand scheme of things.
I'd say adventurers are more like professional athletes in the larger sports. There may be a whole lot of people in society who played ball in high school in the USA (represented in D&D by town guards, common muggers, temple priests and Uncle Bobo who only ever learned Minor Illusion and uses it to entertain the family at picnics), but once you get to the college level there are, what?
Maybe a hundred thousand college athletes out of a population of billions? And once you hit the professional leagues... There are less than 2000 people playing in the NFl and around 450 in the NBA.
Even if there are a significant population of people in a society who can cast the Mending cantrip (or Light, for that matter), it's still not going to wreck (or even majorly impact) an economy for two reasons...
1. Not everybody has access to it.
2. Unless those who do have access to it are under some kind of obligation to cast it for free and on demand, there are going to be very few cases where it wouldn't simply be more cost-effective for the majority of commonly-used items to either repair something the old-fashioned way or even buy a new one instead of finding and paying someone to cast a cantrip...
On the larger issue of verisimilitude, and wizards burning through locks, etc... Verisimilitude is the answer. In game terms, a metal lock is not only going to have a fairly high number of hit points but is also probably going to have resistance to most kinds of damage. Most materials are going to be resistant to at least one or more kinds of damage, even if they're vulnerable to others. And some damage types may not affect a particular substance at all - particularly psychic, necrotic and radiant...
In most cases, it's going to take a significant amount of time for a spellcaster to accomplish something solely using cantrips, and each casting is going to generate noise and highly visible effects.
While a wizard could easily Fireball a door off it's hinges, it's not unreasonable to assume that Fire Bolt isn't going to effect the entire surface area of the door and will at best eventually burn a medium-sized hole in it and maybe set it on fire.
On a slightly different note, a quick perusal of the damage-dealing cantrips in the PHB brings up the interesting fact that out of all of them only Fire Bolt actually specifies "creature or object" as it's target rather than creature, so take that as you will.