Is that simply not a factor in deciding between AMD and Intel in November of 2019?

This. AMD might come out with a Ryzen 3000 minor refresh along the lines of Zen 1 to Zen+. Intel 10th gen will almost certainly need a new chipset and I think it's in a new socket as well. I would choose based on cores and performance and not worry about future in-place upgrades. Whichever you choose will likely last long enough that you would be into a next-gen motherboard and probably next-gen RAM when you need to upgrade.

There might be one more generation on AM4.

Unless your not building a suitable system for your needs at the start on either platform the build will last longer then the upgrade path.

You would be upgrading a dead platform with a dead end CPU when newer and better is available from both companies.

Is that simply not a factor in deciding between AMD and Intel in November of 2019?

Right now it is not a factor. Any thing you buy now (looking at the high end CPUs) is not going to have a meaningful upgrade in the current sockets.

Thanks you guys. That makes sense.

Now is a tricky time to build new when it faces the recycle bin of history. DDR5 next year, next gen CPU's and GPU's next year. Choice is wait or buy now and make do. I personally think technology is always evolving and nothing lasts forever. Build now and upgrade when you feel performance is letting you down. There are still plenty of PC's out there with Haswell chips, DDR3 Ram, and Maxwell GPU's. If it aint broke....

I am trying to decide whether to go AMD (probably Ryzen 9 3900) or Intel (probably i9 9900k) for a new video editing build

If you build now either will do the job although I err AMD since Ryzen 9 3900X > i9-9900K for just about everything, including gaming.