Ironically, Samurai's disengage and gap closer is something I think would fit the Monk perfectly, from a mechanical and job fantasy perspective. They are light hits, which fits the idea of Monk being a fast and mobile fighter that delivers damage through many consecutive blows, rather than large single hits (which admittedly is another aspect of the job's identity that has been thrown into the garbage in part due to the half-baked Blitz system turning them into better AoE specialists than Dancer). It makes sense that even as they retreat from an opponent, they would get in a hit, a sort of love-tap/reminder that we're still here. It feeds into the original job's need to keep up the pressure (due to Greased Lightning, and now to a lesser extent with Twin Snakes taking its place). It makes me think of martial arts films where the heroes evade the villain effortlessly while still striking their opponents even as they fall back, or how quickly they move in on an opponent to exploit a gap in their defenses only they can see to land a particularly vicious strike.
Thunderclap is just another idea that sounded really great on paper for Monk, but effectively didn't take into account how difficult it would be to use under duress. It also fails to account for situations where groups want to spread away from eachother, therefore making it just another button that is situational at best. Yes, there are creative macros one could use to bounce to a player in question for situations that call for it, but from a sheer usability standpoint, the Dancer's dash is a better execution of the same basic concept.
Thunderclap as a tool for enabling offense doesn't appeal to me either because FF14 is incredibly lenient with melee range. You don't actually need to stand on the targeting circle to hit something, you can be a fair distance away. And frankly, I've never once, in any single piece of content I've ever played in this game, thought to myself as Monk "Gee, if only I could just teleport right next to the enemy that I've targeted after running away from it!" because we used to have Shoulder Tackle to effectively do that already! Me moving away from the boss was effectively my disengage (the thing that the job actually needed some help with), re-engagement with the boss was already a solved problem. By making a button that tries to do both, then tying it targeting something/someone, they challenge the value of Six Sided Trash as "disengage", while also providing a solution for something we already had an effective answer to.
In the previous expansion, Monk had the problem of too many things dedicated to maintaining a self buff, all of which were questionable to some degree. Now we arguably too many buttons meant to enable disengagement, one of which is universal to everyone in the game.