I think people that solely focus on the (albeit inflated) price of the 9900K are forgetting what kind of build this CPU is likely to go into, and that is either high end gaming machines with expensive GPUs or a mixed use productivity / gaming PC.
It's somewhat misleading to use CPU price alone to determine its 'true value' because its only a part of the system cost. Yes, a 2600 is 25% the cost of a 9900K using current prices and can still deliver acceptable gaming performance, but would I pair it with a 2080 Ti? No way, because if I'm spending $1000+ on a GPU I'd want the fastest CPU driving it, or at least a 8700K/9700K if not a 9900K, simply for the 5GHz clockspeeds and high gaming IPC.
For example, just looking at the 'value' proposition of the 9900K in my friends high end gaming build, I believe it costs him close to $2500 and thats not even including the cost of his gaming monitor which he already has, if you include that then the cost balloons to over $3000. Even if you substitute the 9900K for a 2700X, you may save $200 or even $300, but the system is still going to cost well over $2000, and you potentially end up with this situation in games, even at 'CPU bound' 1440P:
Now, show these results to a high end gamer spending $2000+ on a gaming PC, and try to convince them that a 2700X (or 2600) is the 'better' choice, even if it saves them a few hundred dollars. What is the point of getting a 144Hz monitor if your CPU is going to bottleneck it to closer to 100 - 110fps avg in certain games? Granted, not even the 9900K can avg 144fps in AC:O, but it gets a lot closer than the 2700X does. Maybe that's of more 'value' to hardcore gamers than saving a few hundred dollars, especially if they are playing in a competitive environment?
I don't mean this in a trolling way, but AMD's Ryzen 2000 chips perform like a 4 year old 4790K in games according to the charts above - which still isn't bad, mind you, but it's clearly a tier or two below 9900K level (or 8700K/9700K or even 8600K level) and its gonna be a tough sell to convince high end gamers that the AMD is the 'better value' if they value outright performance more than a few % higher price/performance. Not to mention that most high end gamers are probably upgrading from a 4790K/6700K/7700K era machines that are a few years old, and if you are telling them that they will be taking a sidegrade or even a downgrade in CPU gaming performance just to get 'more cores for less' with Ryzen, I think that's going to be an incredibly hard sell as well.
If given the choice, I'd take a $2500 9900K + 2080 Ti gaming machine over a $2100 R5 2600 + 2080 Ti config, but the sensible part of me would probably just opt for the 8700K or 9700K instead, since they basically perform like the 9900K in games, unlike the Ryzen parts.
In a nutshell, the 9900K is essentially an attempt at a 'no compromise' all round desktop CPU that has leading gaming and productivity performance, but its going to cost you a pretty penny. If you can afford one, power to you, but for people that can't, there are plenty of cheaper alternatives to choose from.
Now, maybe, just maybe, people can judge the 9900K on its merits and intended audience, because the 'ZOMG its $600, twice a 2700X!!!' type comments are getting tiresome - it's not like anybody here doesn't realise the cost of these chips, but for its intended market (high end desktop / gaming) its a legitimate choice, at least IMO. As I said earlier, its probably not for 99% of buyers, but for that 1% niche it probably makes perfect sense.