That's the thing, for gamers, it may not be +15%, in some circumstances it may be 30-40% plus.
Ryzen 3000 (Zen 2)is already an INCREDIBLE workstation CPU, and it's a good gaming CPU, but the 3300X shows how much can be achieved and how much extra performance is sometimes on the table just from reducing some of the bottlenecks within Zen 2, even BEFORE the Zen2->3 architecture improvements, remember that latency/bottleneck reduction and the architecture improvements should stack; so in Ryzen's worst case scenarios the improvement may not be an extra 15%, it could be 40 or 50, or 100%.
If you're happy with the 3900X, then that's absolutely great, but there's nothing wrong with people being excited by Zen 3, and if they primarily game rather than use the system for productivity, then why not as it sounds like some of the improvements on Zen 3 will make more improvement there and other low latency tasks than anywhere else.