Well to be fair, the 9900k has dropped almost $100, and pretty recently, but you are correct about the FX, that bombed hundreds of $ in its first year. But performance wise, the 9590 was also king of the amd hill for many years, a large OC 8350/8370 came close, but couldn't beat it, IF you could tame it. Most couldn't and had to downclock into the 8 series OC range.
Right now, the cheaper 10600k is only a few fps behind, the 10700k tops it. And that's not including the bump upto the 10900k family. When 11th gen drops, it should be surpassed by the 11600k-11900k, putting it in 7th place, barely ahead of 8th and 9th place 10600k - 9700k. That's a sizable drop for a flagship in just 2 generations, not even the 4790k dropped as far vs the 6700k/7700k (discounting Broadwell fiasco).
I just don't see the 9900k really holding good value in the long run. Supply will far outweigh demand. Who'd buy a used 9900k with all its thermal issues when it's beaten by a cheaper platform with half the needs? Upgrading now, sure, I can see that, plenty of impulse buyers dumping the 9900k in favor of a 10900k, but in 5 years? Doubtful. They'll be a dime a dozen. They'll be what 3rd, 4th gen are now except probably not in the Indo-Asiatic markets since those boys already have summertime heat issues.