Use Cases for an External ADC
If the internal ADC of your microcontroller performs the job you need it to then no, there is no need for external ADCs. But then, that's not who they're aimed at.

You have covered most of the reasons for an external ADC, but there are a few more, and in my opinion, they are some of the most important reasons:


You need a different sampling technology - for instance the internal ADC is SAR, but you need to do Delta Sigma.
The internal ADC, because it is internal, and shares the same die as the main MCU, will never be 100% free from the noise of the rest of the MCU, so an external one would be possible to make ultra low-noise
Your microcontroller / SoC / FPGA of choice has no ADC.  The latter two are most likely - most common SoCs and FPGAs don't have any ADC at all.  Yes, you can get ones that do, but many don't.  So you add an external one.


For point 3, take the Raspberry Pi for example.  That has no ADC available at all, you have to add an external one to do any analog work at all.