Depends what you are into I guess, for me moar cores is always better - video encoders, path tracing renderers and compositors are my main compute workloads of choice.
Mine is mostly VM work and faster compile times among other things. $500 for the 3900X is a lot of money, but it does that work lickity split, to quote an older coworker who scored one for home use. You could always wait in this era of AMD where each gen gets 8-15% IPC increase along the benefits of a refined or mature node, but you may get stuck waiting "indefinitely." After Core matured in the 2010s, particularly with Sandy Bridge, you didn't need to update for a long time. On the gaming front, they, AMD, have made decent progress. There's a guy on youtube called thespyhood who's done generational comparisons of the most popular AMD Ryzen processors and there's a 15-20 fps gain on minimums and averages for most games.
I suspect Zen 3's single-thread performance will be greatly improved to match Intel or nudge past it. I suspect mult-thread performance to improve yet again. I expect better clocks out of the box, better boosting and all the other typical goodies people are theorizing about. My next upgrade after that will probably be Zen 5. Just because Intel hit a homerun with Core, it doesn't mean their next-gen uarch will be amazing, too. The amount of bad uarchs in history outnumber good ones. Keller is an excellent engineer, but placing too much emphasis on his work when there were more hands on people involved with Zen, than him, is a disservice to all the others. However, Keller isn't a messiah who will solve Intel's problems. Intel's next gen could have a 40% IPC increase over today's 10th gen parts, but it could in 2022-2023 match what AMD has in plan, or fall short. In other words, it may workout to Intel pulling their own AMD dark period.
As far as Mama Su/Dr. Lisa Su goes, she's an excellent leader and very likely a skilled engineer herself. But let's be real, she and the rest of AMD's management are out for Intel's blood, and rightfully so. They're probably going to pummel Intel for years.