The new reality is that the latest TCs, internally and external are excellent optical quality, and top wildlife and sports pro photographers around the world use them all the time, with excellent results.

Just go to any Wildlife and Sports shooters groups and forums and see wonderfully sharp and optically excellent shots made with the latest TCs. It is just fake news that TCs are no good nowadays. The latest TCs and best long lenses are designed to work great together, and they do if you know how and when to use them.

Here top wildlife pro Mark Smith uses both Sony 1.4X TC and Sony 2X TC with the latest Sony 300mm f2.8 GM lens:

Sony 420MM F4? What???? 300mm 2.8 + 1.4 Tele - Beautiful Combo for bird and wildlife photography.
https://www.youtube.com/watch?v=w4WA2OglKLo&t=90s

Sony 300mm 2.8 + 2x Tele - Beautiful Combo for bird and wildlife photography.
https://www.youtube.com/watch?v=XBNF3xu5w5w&t=436s

Here my 24mp Sony A9, Sony 200-600mm lens and 1.4X TC at 840mm, as kids get pulled by a fast speedboat on Brookings Lake, Manistee National Forest, Michigan, USA. This is a 50% crop in here.

Click on download to see better image quality, even with UHH photo compression here. No noise reduction has been done in post on this image, but I could clean it up a bit with my Topaz Denoise AI if I choose to.

Cheers and best to you.

I will add some to my earlier post above. Back wen I was shooting with slide film, I often used a Nikon 500mm f4 P lens with both the Nikon TC 14B (1.4x tc) and the TC 301 (2xtc) attached to my film camera. Resulting images were useable and acceptable if the whole rig was adequately supported (usually with two supports) and the shutter was triggered with a cable release. Focusing was done manually. Support and technique was critical.

Now that I am shooting digitally, support and technique is still important for good IQ images both with and without tcs and with and without cropping. My personal preference would be to add a tc before cropping in post. However, if good support or technique is not possible, then I would likely not add the tc and just crop.

I have to admit these appear sharper than I ever would have expected by stacking TC's.
Usually results suffer significantly.
Now a Nikon 400/2.8 will be an exceptionally sharp lens, and the fact that the 1.4 is built inn means it is perfectly matched to that lens, so it is not surprising that that combo will be sharp.
Adding an additional 2x seems to have worked fairly well. I can't tell about critical sharpness but it looks good.

One thing it proves is that starting with a great lens is really important, followed by optical matching of the converter. There does seem to be a reduction in contrast, but I don't know if that is optical or processing.
Interesting

I agree 100% that the foot on the Nikon lenses these days are way too small though personally I would prefer that they are Arca-Swiss compatible. But even if they had the Arca-Swiss grooves on their feet, not sure I would use them. I can barely fit 3 fingers on them (and I have small hands). Plus they have the button to rapidly release the lens if the knob is not fully tightened right there and because the foot is so small, one is constantly touching it (in the picture below, my finger is pressed right up against the rapid release button). So a disaster waiting to happen. To me, the foot on a Nikon lens is a bad joke. One could never balance it on a gimbal head. Whoever designed these feet would seem to have never used them. This is the foot that comes with the $3200 Nikon 400mm f/4.5 lens: