I can't see upgrading from my 8700k at the moment. The 9700k and 9900k just don't seem to be a big jump according to everything I have read. I may look at the new Ryzens once the official benchmarks come out.
It completely depends on what you're doing.
The 9700k has 8 cores and runs faster than the 8700k - if you're using single threaded software, or up to 8 threads, it will be quicker than the 8700k by a margin. It's when you get to 12 or more threads that it's within 10%
The 9900k has 8 cores and 16 threads, and runs faster than the 8700k - It is about 20-25% faster than the 9700k in applications that can use the threads (eg productivity/content creation), which in turn is about 10% faster than the 8700k, so 35-40% faster overall for this sort of software.
For me, as a productivity user, the difference depends on the software I'm using. Compiling uses all of the threads you can throw at it and more compiles means more time to debug, faster single thread performance means that the bits that can't be multithreaded are faster.
In CAD you can get more renders per minute of compute with more threads.
In real world terms for AutoCAD inventor:
If an 8700k will give me 1000 passes in 5 minutes,
A 9700k will give me 1100 passes in 5 minutes
A 9900k will give me 1400 passes in 5 minutes
This means for the same 5 minutes, I can end up with a 40% superior render, or spend 40% less time doing the render to get the same result.
If I'm doing a job which requires me to do a demo, being able to spin up a render quicker gives me more time to focus on other things.
Those other things, in my case as a contractor, include spending more time with family, more sleep (8 hours), or spending time with friends all of which are very important.