Well, the basis of it is semi realistic, so I sure as hell wouldn’t object to it as DM. It’s the rules working just like they’re supposed to, imo.
There is a functional, real world difference between disarming someone and pickpocketing them during a physical encounter. You couldn’t use sleight of hand to disarm anyone, ever, because the thing is in their hand. Just like you can’t disarm something out of a pocket.
But there’s no reason you couldn’t take someone’s sword off their hip. Ever see those tethers cops use? Well, used to use. They use holsters that make it harder to snatch them now. Point being that a weapon that isn’t in your hand is vulnerable to sneaky grabs, or even non sneaky ones from a skilled attacker.
A pouch of components, that’s no different than a coin purse unless someone takes steps to make it so.
Here’s the thing though. Not all targets are equally difficult. Someone sees you coming, and you’re an enemy, they aren’t going to just stand there. The act of moving into their space changes it a good bit. If they’re also skilled in a similar way, i.e. having levels of rogue, they’ll recognize what you’re trying. A fighter isn’t going to keep a dagger just loosely hanging at hip, and let you reach for it. You might have to grapple first, then make the grab. Or maybe a touch attack equivalent. It isn’t the same as lifting a coin purse in a crowd, there’s a higher level of difficulty involved.
Admittedly, with a 19 roll, you should have succeeded anyway, once any other criteria were met.
But, yeah, in principle, that’s a totally valid idea. But it’s also relatively easy to counter, and it isn’t even unusual. If a component pouch is on a belt and firmly attached, that isn’t sleight of hand any more. Auto fail to try and grab the pouch and run. You could empty the pouch, but not take the whole thing. You could maybe call it a sunder attempt, but that’s still different.
A 20 foot dash is maybe three seconds, four from a dead stop. While you can get to someone before they can draw a weapon and hit you with it, it’s still a lot of telegraphing to overcome. An experienced thug is expecting trouble, and would have some variety of unarmed combat in their kit, so it wouldn’t be exactly as you described it, if the DM is thinking a little. But it’s within believability.