IIRC from our previous discussions, your group likes to not only optimize their characters but rather optimize the party itself. The game isn't calibrated for that.
Yes - many of the stunts my players pull off doesn't seem to be that impressive on their own - but as enablers for other party members they're excellent.
Monk Stun is perhaps the best example. The Monk hasn't killed that many monsters by himself. But the player is more than happy by his contribution.
But I need to talk about a serious issue: "The game isn't calibrated for that."
On the surface of things, this is not only completely true, but also as it should be - expected even. No edition of D&D (its monsters, adventures etc) has been or should be geared towards the optimal party.
But this statement is easily relativized:
Since the game shouldn't be calibrated for "that", everything is okay and nothing needs to change.
This is deeply wrong. 5th edition is
noticeably weaker and
less challenging than any other edition of the game I've encountered. It is so very soft that
it becomes a problem, since no longer is it enought to just beef up encounters to make them provide enough challenge.
5e is the first edition where encounters must be
wholesale replaced from scratch, and this is a
huge failing of the edition.
(The reason is a double whammy: not only is monsters simplified beyond any reasonable limit and have lost essential survival tricks, but 5e characters are given a number of extra gimmies, lives and tricks far beyond that of any previous edition)