It basically got pushed out by android as even very low end devices can now mostly run android sort of ok, right?

I was looking at the phone mainly to use it as a 4G WiFi hotspot.

What is wrong with KaiOS itself?

KaiOS is struggling because business on the low-end segment is very hard. Which means that the technical side is cutting corners and you end up with a lacking product in important areas like security. (disclaimer: I worked there and left recently)

It's not great at all. Convicted persons on parole are often disallowed from Internet use or smartphone ownership, so I see a lot of parolees with KaiOS flip-phones.

The text entry is really, really horrible. That's probably the worst bit. The OS is pretty unintuitive and inconsistent in the UX. No-one is really developing apps for it as far as I know.

It's definitely way worse than c. 2007 Nokia Symbian devices.