ph0rk
Friendship is Magic, and Magic is Heresy.
So, I've not played a thief but I have messed around with caltrops and battlemaster maneuvers. These grow stale; particularly by Tier 3. I'm not saying they aren't better than a champion or samurai - they most certainly are, but the list won't ever approach any particular wizard's list, not will they have the "what do I prepare today" moment, either.5e doesn't make it easy, but with multiclassing, feats, and careful subclass choice one can make a martial with a complex decision tree, rivalling that of a full caster.
In particular, any martial with 3+ levels of Thief and a large supply of consumables (oil, alchemist's fire, caltrops, acid, the DMG poisons, etc.) can easily have a dozen or more options for what to do with their bonus action each turn. Sure, only a few will be relevant in any given situation, but that's just as true for casters' spells. Combine the Thief with the built-in choices of a Battlemaster or Echo Knight and the round-by-round options start piling up. And if that still isn't enough, feats with in-combat options like Healer can add even more decision points.
There are whole threads where people gripe about the sorcerer relative to the wizard for this very reason - it is the flexibility, more than particularly explosive power that they want (as a sorcerer actually has just a bit more of that than a wizard).