Short version: adding enemies and tactical play on the part of the GM can keep TN (target numbers) going no higher than 15 and still having great challenges.
Long convoluted diatribe below:
Please explain DC? Do you mean TN? Or damage class? Defense class?
I suspect that it is Difficulty Class. D&D terms are not universal and even what version of D&D can make it odd. Many of the people here know D&D well but acronyms typically trigger based on the game they are playing now.
Anyway, your assumption is a bit wrong. I played in a high powered session of AS using loot from any of the books available at the time. (We had a limited time to make characters so we did not totally optimize…but we where all pretty stout) @Ezzerharden ran the game.
In AS character survivability was way better than I expected, and at the high end, it was the characters mobility that really showed.
As to needing to raise the (Target Number) TN, not really. Characters have 10 active loot max. And you are rolling a D20. So assuming a min/max archer (speed quiver) character with +11 dex and +5 to magic damage With a magic bow, 15 AC and 3 hearts. Will Probably have a 75/25 shot with 30 goblins and a TN of 12.
The archer needs to roll a 2 to shoot again and to hit. The average hit will do 9 damage to 10 hp goblins.
The goblins have to roll 13 to hit, and averaged damage of 5.
Assume after archer turn 1 10 goblins are left. 4 will probably hit for 20 damage.
Archer then kills the 10 remaining goblins.
That’s the outcome assuming perfectly even rolls. An early 1 and the archer is toast.
Goblins using tactics, limiting potential targets…and this becomes a long drawn out (6 rounds)with advantage to the goblins.
Switch out for a couple of goblin bezerker types and shamans…it won’t be too much of a long fight and the archer is dead.
So, modify play to keep it interesting and don’t use TN as a crutch for high end play.
And the Links there might inspire added insights.