This is an incomplete though common on the web point of view.
You perhaps overlook you get 1 stop more depth of field using a TC than using a longer focal length without a TC.
As an example with the 100-400 at 200mm f8 if you zoom out to 280mm the exposure stays the same at f8 - and because of the increased subject magnification you get half (2 stops less) depth of field.
If instead you start with the 70 - 200 at f8 and attach a 1.4 TC (a not uncommon combination) to get a 280mm angle of view the aperture readout (see NOTE 1) is f11 together with a 1 stop longer exposure time with - with 1 stop more depth of field than using a longer focal length at 280mm without a TC.
If you open up the 70-200 (with 1.4 TC) by 1 stop to an f8 exposure time readout you get the same f8 shutter speed and the same reduced depth of field as with the 100-400 at f8 without a converter
When not starting from wide open and adding a TC you have the choice of the same shutter speed with 1 stop more depth of field than with a longer focal length without a TC.
Next - cropping by the equivalent of 1.4 TC reduces 45 MP to 31.5 MP on the subject - with an inevitable reduction in image resolution - a potential decision detail.
My experience with F mount lenses and a Nikon F 1.4 TC was that with first class technique more image resolution was usually possible than equivalent cropping.
It seems widely accepted Z mount lenses that take a Z TC take them distinctly better than F mount lenses and F mount TCs.
I find this to be true - with a greater image resolution gain with a Z TC on a Z lens than an F TC on F lens - compared to equivalent cropping.
Loss of viewfinder brightness with a TC is a negligible issue with ML due to the viewfinder starting several stops brighter than with the F mount.
For those who cannot afford either the 120-300 f2.8 or 180-400 f4 to get from 200mm to 280 without a TC and with a relatively fast aperture - there is no free lunch
There can be marginally slower AF though less slower AF than going from a sunlit scene to shade.
There can be marginally higher noise.
1 stop more noise is usually marginal, easily handled in post and the negative effect is less on image resolution than cropping 45 MP to 31.5 MP instead of using a TC.
My advice is do not be afraid of using a TC!
NOTE 1.
A 280mm at f8 requires a physically bigger aperture to be f8 than a 200mm f8.
When extending a lens from 200mm to 280 with a 1.4 TC the size of the aperture does not change.
The physical size of an f8 aperture at 200mm is close to 25mm (200 divided by 8)
When going to 280mm angle of view with a converter the physical size of the aperture remains at 25mm.
280mm divided by 25mm is within a whisker of f11 for depth of field purposes.
