Minor nitpick: 8K will also be largely irrelevant in cinemas! At least near me, many of them still broadcast 2K on most of their screens, with maybe one or two set up for 4K projection. Only the most eagle-eyed audience members could even theoretically notice a difference in 4K and 8K projection at a viewing distance where they could see the entire screen at once - and outside of maybe a nature documentary, even among those viewers, it's hard to imagine them caring much.
Not true.
Aside from digital zoom without substantial quality loss, as you mentioned, a number of VFX use cases, especially those involving masking something out, are improved by having the highest-resolution source material available.
That and in theory, you'll get better color information putting 6K or 8K on a 4K timeline, assuming the use of a Bayer sensor. Remember that it's necessary to guess at the values for 2 out of 3 colors for every pixel when debayering footage. From what I remember, and I might remember wrong, this means that the readout from Bayer sensors is effectively 4:2:2. Downsampling from 6K or 8K gives you something that is more effectively 4:4:4.
As to whether that matters to you, maybe none of it does. In practice, modern debayering is very good and I'm not really sure that viewers are ever going to notice a difference in 4:2:2 and 4:4:4 once at 2K resolution or above.
Correct. A 4.6K Arri is still preferred in many productions to an 8K camera from another vendor. The FX5 is going to sell a million bazillion units despite being released as a 5K camera in an era when all of the competitors are at 6K or higher (also assuming that some of the firmware weirdness and constant rebooting to change settings can be fixed). There are still cameras being released today using the same IMX410 that was in my Z Cam E2-F6 that I bought in 2018.
The Ronin 4D was one of the best product ideas that DJI ever had (really, the Osmo Raw was, but the execution was too clunky). They were never going to compete with Arri in the space where people use Arri - so take a real pain point from a lot of productions (gimbals are irritating to use/balance and are often disconnected in operation from the attached camera) and fix that. Now you see the 4D showing up in all sorts of productions - because it's the best camera ever made and wins the spec sheet wars? No. But the quality is good and it's an elegant solution for some problems.