First, 15 points can get you a lot in GURPS. You could add +4 to a skill you have. You could get +1 to all defenses. You could get a reroll once per hour. You could get +3 to a more narrow group of skills. If you save 5 more points, you could buy a level of IQ and add +1 to a vast swath of skills.
Second, Smooth Operator is barely a 15-point talent as it is. It has 13 skills, which is just barely the threshold for a 15 point talent. It has two skills that are HT-based, One that's Per-Based, and ten that are all based on IQ. That's some reach across attributes, but not a lot. Its reaction bonus basically says that if the person matters, the reaction bonus doesn't apply. Its not as narrowly focused as some talents, but it clearly represents a very coherent set of skills. Despite this, I struggle to imagine a character who actually uses all 13 skills, because several of them are pretty niche in application, like panhandling or politics.
Third, the Basic set acknowledges that "the GM’s word is law when determining which skills are “related” and how may points the Talent is worth." This is for custom talents, but its also a general caution that there is a "Feel" to designing properly balanced talents, rather than just relying on counting skills.
So I don't feel bad telling people not to take smooth operator. And that indicates its not worth those 15 points. Is it worth 10 points? probably. It still faces some pretty stiff competition from options like Charisma and the Talker Talent from power ups. So even at 10 points, its not an obvious choice for the type of character that its designed for. And that, to me, is the best proof that lowering its cost to 10 points isn't broken: that it doesn't create a rush of players trying to take it.
So go ahead and try pricing smooth operator at 10 points. Hopefully it makes a certain type of character more viable, or at least less awkwardly built. Happy Gaming!
