CHG_CANON Loc: the Windy City

You're confusing a number of different technologies leading to a misunderstanding of the possibilities ...

1, DSLR and MILC technology are different on several fundamental technical ways. There are no AF point limitations for mirrorless camera with smaller and smaller effective apertures. The entire frame is active and available, regardless of the lenses and lens+extender configurations.

2, The RF extenders have no zoom focal length limitations for compatible RF lenses.

3, The EF extenders have no zoom focal length limitations for compatible EF lenses, although some EOS DSLRosaurs are limited for center-only or a smaller total number of AF points when the effective aperture reaches f/8.

4, The autofocus of EOS mirrorless cameras are not limited to f/8 like DSLRosaurs.

SuperflyTNT Loc: Manassas VA

As you know, I’m not a Canon guy, but I was gonna make the same point about focus points not being limited and all focus points working beyond f/8. The other issue about focal length limitations seems to be a real thing. On the one hand it’s true that most people using the TC on the 100-500 will be using it at the long end most of the time, but being limited to 420-700 instead of 140-700 is losing a lot of the flexibility of using a zoom in the first place.

DRM Loc: NC

It's only limiting if the TC is already attached for long use and suddenly a need arises for a much shorter focal length . . . wildlife suddenly appears much closer than is the norm, for example. In those cases, the design could indeed be limiting. But back to the OP's original post, he (or anyone) would potentially be somewhat similarly limited with the 200-800.

Depending a bit on the R-series camera attached to the 100-500, foregoing the TC and cropping as necessary/desired might be the best approach in most situations. Not all, perhaps, but most.

CHG_CANON Loc: the Windy City

I will note that correction and register the disappointment in Canon not even footnoting (*) this issue in their compatibility references.

MountainDave

I have found limiting the use of extenders to the 300-500 range does limit its usefulness quite a bit. It's also more awkward to carry. Given Canon's excuse for not making the 70-200s and 135 compatible with extenders, I assume weight is the reason here as well. I'm OK with that. The size and weight of the 100-500 is a major factor making it a joy to use.

SuperflyTNT Loc: Manassas VA

Like I said, most people are using it mostly at the long end, but it does effect the flexibility quite a bit. The 200-800 is a 4x zoom. The 100-500 is a 5x zoom. With the 1.4 TC it’s effectively a 1.67x zoom. And at its widest it’s more 2x the 200-800 at its widest.

SuperflyTNT Loc: Manassas VA

I’m sure that’s the reason. Also the reason it’s an f/7.1 at its longest. They wanted to keep it as compact and light as possible. It appears much smaller than my 100-400 Z lens.