Correct me if I'm wrong, but one would be better off getting a 2700x for roughly the same price as a 1900X, newer and about 5% faster overall.
Well it depends on what the rig is being used for, but yes, that is a correct assumption. The only reason you would consider a 1900X at this point is to provide the upgrade path for the 2990X.
I'd wager the 1900X wouldn't be worth it coming from a 4700k (4.2 mild OC).
I'd say you would wager correctly. Ryzen 2nd generation is a far better investment.
I see where you are going, I really do - that could be a good idea where it not for the 1500$.
Forgive me for being out of the CPU loop so to speak but I would like to challenge the "core count and lots of TDP equals better" idea a bit. What if 8 cores are enough with lower TDP and higher clocks?