[Brightcolours]

4.5 mts MFD is another disappointment ...... max mag. 0.14X ... rules out most insect photography .....


Tamron G2 

MFD 2.2 mts ...... max. mag. 1:39X ... great for butterflies etc.

  The 800mm F11 ........ 
 .......seems a more realistic proposition and has an MFD of 6 mts with a max mag of 0.14X ...... not great either but there's no other alternative in the light weight division .... 

Another point to take into consideration, dave:
Yes, because they are compact primes, they apparently do not have the room for a longer focus path (so a shorter MFD), so they behave like normal 600mm and 800mm primes (Nikkor AF-S 600mm f4 E FL VR : MFD 4.4 meters, max. mag. 0.14X. Nikkor AF-S 800mm f5.6 E FL VR: MFD 5.9 meters, max. mag. 0.15x ... looks familiar?).
And yes, the Nikkor 200-500mm Sigma 150-600mm, Tamron 150-600mm all get closer and get a higher max. magnification. But, they are not 600mm at MFD. They all show similar widening focus breathing towards MFD (and according to the specs, so will the new Canon RF 100-500mm L).

So, yes, you are right, the zooms get closer and the zooms get higher magnification. But they also do not offer 600mm narrow FOV, not even close.

I have not found any focus breathing video of the Tamron 150-500mm lenses yet to illustrate the focus breathing, but I did find a comparison of "one of the supertelezooms" (so either Tamron, Sigma or Nikkor) with the Nikkor AF-S 500mm f4 you are very familiar with (which does MFD 3.6 and 0.15x max. mag.) :
https://photographylife.com/nikon-200-50...50-600mm-c
This then is at least 3.6 meters away, the widening of the FOV will of course get worse at MFD for those super zooms. 

So if you are after the narrow FOV for close ups, the super zooms won't offer that.  If you want that narrow FOV 600mm would give, you will have to use extension tubes with the zooms, even more than with the primes.