My friend had 24-70 f4 IS and he liked its sharpness and IS. According to him F4 was a very good lens but not the best. He upgraded to 24-70 f2.8 II version and since the upgrade he's so impressed with the f2.8 version he sold all his primes: Sigma ART 35 f1.4, Sigma ART 50 f1.4, and Canon 135 f2. The reason he sold Canon 135 is because he also bought 70-200 f2.8 II. Both Canon f2.8 zoom lenses are excellent that you can forget changing it to prime lenses. I'm going to upgrade my 24-70 f2.8 I to the II version.
It's a great zoom, don't get me wrong, but the 24-70 II's reputations of 'it lets me leave the primes at home' is only clearly true when you are looking at ordinary L primes from a sharpness perspective, like the older 35L (I) or clearly-not-built-primarily-around-sharpness 50L.
But stack up a modern
prime against that 24-70 L II and it comes back down to earth: the Sigma 35 Art at f/1.4
outresolves the 24-70 II at f/2.8. I'd imagine the same would be true of the 35L II.
Also, it's modestly challenging to shoot that 24-70 II at f/1.4.
Don't get me wrong, the 24-70 II is a stellar piece of gear, but an equally well designed/toleranced/built prime can do things that the zoom can't.