On what planet are these garbage?
(Other than Defensive Duelist not really being worth it until Lv. 8 or higher, but you could say the same for Resilient.)
Assuming AC is in bounded range, a +4-6 has a X=20%-30% chance of being useful each attack.
At 4 attacks and 30%, you get a ~76% chance of using it each round, blocking ~19% of the potential damage.
At 3 attacks and 30%, you get a ~66% chance of using it each round, blocking ~22% of the potential damage.
At 2 attacks and 30%, you get a ~51% chance of using it each round, blocking ~26% of the potential damage.
At 1 attack and 30%, you get a ~30% chance of using it each round, blocking ~30% of the potential damage.
I think high level monsters more rarely have single attacks. And at high levels you fight such monsters more often.
At low levels, you both have few attacks and your chance is low per attack; so it is less useful. But I think it does plateau.
I suspect the 2->3 transition, where monsters typically go from 1 attack to 2, is a wash.
Then, at high levels, threat coming in the form of saves starts ramping up. So by the time you get a high +6 modifier, lots of the fights it is near useless.