In my opinion, if I was to build a strictly gaming/browsing rig, here's a couple things I'd consider:
1) 9700k is really the only current CPU from Intel that fits the bill. 9900k is a waste of money (especially adding in cost of a suitable cooler) unless you do some content as well. Its just too expensive.
Pros:
- can get slightly higher FPS at 1080p on 2080 series cards
- can OC to that satisfyingly even "5.0" number in most cases.
Cons:
- Intel platforms leave pretty much no chance to only upgrade CPU - you'll need a new mobo as well when you want to upgrade
- generally will cost more
2) 3700x is probably the equivalent CPU you want from the AMD side, although the 3600 is a fine cpu as well. Either of those will fit the bill for your needs.
Pros:
- less $$$ for 95% of the performance in most games
- a chance (not guaranteed) that you will be able to put a future CPU in the same mobo and save some $$ for other upgrades
-stock cooler is generally good enough
Cons:
- lightly less max fps in some scenarios (2080 series gpu - as you said you will have)
- can't really OC the CPU
Other considerations:
You can save money with Ryzen and buy faster RAM, tweak and OC the RAM and get up to 20% performance increase in some games. Tweaking RAM is a pain but there's tools out there that take the work out of it and on Ryzen the difference is quite pronounced. I see RAM tweaking and OCing replacing the CPU OCing with Ryzen, and the gains are worth it.
If you have or get anything less than a RTX2080, then the 9700k will lose its advantage. It takes a pretty clear CPU bottleneck to even bring the CPU into the equation. You said you will move to 1440p gaming - this will also bring any small difference between the 9700k and say a 3700x pretty much back to parity. Will it be worth the extra money to you to have a few FPS advantage just for the duration while you are playing 1080p on 2080 series card?
What money you potentially save on cooling and Ryzen, you can put back into faster SSD, or GPU. You can also put that money back into better RAM as well - again helping the Ryzen out in games.
If you already have a great cooling solution, then factor that in as well, that will add value to a 9700k decision.
Also learning curve for the new platform especially if you like to tweak and OC ... if learning something new is fun to you, this should not impact the decision, but if you hate it, familiarity with the Intel system may add value for the 9700k.
I personally would lean to Ryzen for longevity reasons ... 12 or 16 vs 8 threads will add future proofing as games get programmed to handle more of them (which is already happening); I don't think AM4 will last two more generations making a better upgrade path, but you never know. I have first gen Ryzen and will very happily be upgrading to a 12 core in the same mobo very soon, so that has had a lot for value for me, but as I said its not guaranteed. Monitors at 1440p and 4k will get cheaper and most people will want to play at native resolution - again the longevity here will even the small difference out over time.
If by chance you end up with say only a 2070 or 5700 series, again the GPU will become the bottleneck and the difference you see in gaming benchmarks in reviews won't be the differences you experience. Note that those always use 1080p, 2080ti, and not the highest graphical settings - the numbers you see relate to
this scenario specifically - anything different and the difference will be less to none.
If in the future you by chance find yourself needing more cores and threads for some new thing you discovered you like to do with your computer (like streaming), then you'll wish you got the Ryzen if you didn't.
Its close, but for the longevity aspect I'd go Ryzen. But personally that is partly driven by my desire to own a CPU for at least three+ years before upgrading anything ... if you are the type to upgrade as soon as new stuff comes out and have the $$ to do it, then that might add a little value to any other value proposition the 9700k has.
Either way, there's no wrong choice, just whatever you think will suit you best.