Flint is their newest / highest end router, has about the compute perf. of Pi3, but has WiFi 6.

The lagging customized OpenWRT release cycle worries me. My router is the thing I most want to be up to date with patches / fixes.

You can flash upstream owrt to then. You don't have to run the vendor firmware. I buy GL.iNet devices and the first thing I do is flash upstream.

wow, these look indeed great. I hope I remember these when my current router dies.

Why are some of their devices EOL if they use OpenWRT?

Because companies don't make the same product forever.

Why aren't Ford making the Model T anymore? Why aren't Viewsonic making CRT monitors anymore? etc

Of course I didn't mean hardware EOL, that I understand, but why discontinue software updates then?

Because they're a business, not a charity.

They also do this the right way and upstream device support before they sell the device, so they don't have to support it. Upstream OpenWrt are already doing it.