I also took stock of DTP to gauge the sharpness and you seem to agree that the 300L IS looks no sharper than the new 100-400L IS. To me that is unconvincing performance.
Then the 100-400 mark II,
yes. It's quite a bit sharper then the mark I.
What was "excellent" prime performance yesterday is not "excellent" when new tech produces ever better lenses.
The mark II isn't better, though it pulls even. You're assuming that for a given level of lens design/manufacturing tech that primes should always be much better then zooms. That's a false assumption.
None of Canon's primes, not even their most expensive and newest mark II super telephotos, are significantly sharper then the 300 f/4L IS OR the 100-400 mark II. Their most recent L zooms...the 70-300L, 100-400 II, and the latest 70-200L variants...quite dramatically closed the gap on all of their primes vs. the situation we had just a decade ago.
The same is true even for the Sigma ART and Zeiss Otus primes. Sharper then the L zooms? Yes. But by small amounts.
The way you described the 300 f/4L IS someone might think it's mediocre and that a new version would be dramatically better. Old design or not, it's one of the sharper lenses in existence, and you have to look at the very best primes made to find even a slight improvement. At the level we're debating IQ gains are very small and very, very expensive.
That does not mean your 300IS L takes any lesser pictures than yesterday. But it shows that an upgrade could be even better. Why be troubled by that?
The gain seen in the 100-400 II does not mean an updated 300 f/4L IS would be much better. A perfect lens is diffraction limited wide open. If you look at the 300 f/2.8L II it's roughly one stop away from that right now. f/4 is better then f/2.8 (barely). There's no gain at f/5.6, and f/8 sees degradation. I don't expect a new 300 f/4L would do any better then this, and this is a relatively small gain in IQ vs. the current f/4L IS.
2) 70-300L IS II (which I also have) is only OK and cannot compare with the 300mm L IS
The edges are better on the primes, with mid-frame and center being only very slightly better (f/4L or f/2.8L II). But the high contrast TDP charts emphasize any differences. Given a real scene...good post processing technique...you would be hard pressed to pick unlabeled prints from one or the other.
Doesn't mean I wouldn't pick one of the primes. Aperture is aperture and even a single stop can be huge. But we are splitting hairs when debating the IQ of any of these lenses. EF-S 55-250 STM vs. one of the 300L primes...THERE'S a big difference, easily print visible, no post processing away that. 70-300L vs. one of the 300 primes? Eh...the Ferrari does 185 mph, the Porsche does 187 mph, either one will get you a nasty ticket