4 is more than one and it’s a much simpler game so doesn’t need to use 6 or 8 cores.
Doubt it. It would have to be a perfect scenario of not being GPU, bandwidth, network and WoW server limited. So you go to some secluded old zone where there is no one else and maybe there you could see CPU effect on the FPS which would be 200+ if not more. In more realistic scenarios there is way more performance bottlenecks this game can show. Even top of the line systems can show odd performance problems, quite often after a new major patch 
Good but not groundbreaking. The 2000 series are still here and are heavily discounted. If the OP is on a tight budget picking double-discounted XMass Ryzen 2000 could be much better value (where as juicy 3000 chips will likely keep high price due to demand and XMass sales). If the price isn’t critical then yes 3000 would be a good choice but it won’t make or break the PC.
Some are still using Sandy Bridge CPUs and don’t feel the need to change. It’s all about how much you want to spend and how often.
Define “very low” as IMHO you are exaggerating by a lot. All of Ryzens have modern single threaded performance: https://www.guru3d.com/articles-pages/amd-ryzen-7-2700x-review,9.html which in conjunction with low requirements of WoW allows people to play with lower tier hardware. Most popular steam GPU is still GTX 1060 and so forth.
The more budget you have the more expensive parts you can buy. The question is does 3600 is still within budget of the OP alongside acceptable GPU? Also did you measured your framerates in different scenarios, 1% lows etc?
The question still stands. Is upgrading from 120 FPS to 140 FPS worth the extra cash and do OP has it? What if OP has a 60 Hz display and will cap WoW at 60 FPS anyway? What if that 20 FPS upgrade requires much better GPU than the OP will have?
Also do note that in 2020 we will get Ryzen 4000 with yet another improvement. You would have to upgrade your PC yearly to stay at the “top”.