No sense in upgrading right now, the 9700k and 9900k effectively offer the same gaming performance. At stock, the new 9th gen chips are ever so slightly faster due to higher stock clockspeeds, but that advantage effectively disappears once you overclock the 8700k to the same speed. The 9th gen chips aren't really overclocking much better than 8th gen, and games aren't currently benefiting from the extra cores and threads, so you wouldn't see any real performance uplift unless you are also livestreaming, in which case the 9900k would help there.
now I'm using 8700k on Z370 mobo, I use my PC only for gaming purposes and network surfing, nothing else.
I'm playing on 1440p / 144hz.
Is it worth the upgrade or not?
No, you should not upgrade that system unless you just have money to throw away for a small gain in performance. I also have an i7 8700k/GTX 1080 Ti/ 1440p system and will not upgrade. But I will get an i7 9700k and new motherboard, RAM, SSD to upgrade my son's 7-year-old AMD system.
So, if you have a need to build a new PC just for gaming and wanted the best CPU to do that you would get the i7 9700k. Those who do content creation and rendering or game streaming/recording as well would go with the i9 9900k.