I've not tried the new 70-200/2.8 Z, but I am very familiar with the older EF 70-200/2.8 II counterpart. It was an astonishingly sharp lens even with a 1.4x tc. It af our resolves any of the current Canon sensors. The 2x less so, still sharp but want's great with close objects and generally needed stopping down 2/3 stop to remove the slight haze around highlights. The thing is, a pair of teleconverters is a lot lighter than lugging around another sized lens. Even the 2x TC works well if that's the only thing you have on you. I appreciate that this new RF Z version is a tad sharper again, but it's still in the same ball park with teleconverters.
The RF 70-200/2.8 is a miracle in packaging and light weight construction by comparison, but we all know of the lack of TC support for this sweet lens.
The RF 100-300mm f2.8 is a very different beast. With tele converters, it's pushing into wildlife and birding reaches. Even with a 2x TC (making a 600mm f5.6) it just about keeps up with the R5's resolution of it's mighty sensor. It's a very viable and versatile lens range (100-300/f2.8 > 140-420/f4>200-600/f5.6) and it only cost you the size and weight of a pair of teleconverters. It's never going to compete directly with a 70-200mm f2.8 even though it's got a fair amount of overlap in the focal range and aperture. The Rf 100-300/2.8 is kind of a bridge lens between the 70-200/2.8 and the longer wild life lenses.
I have a few friends who regaulrly use a EF300mm f2.8 II LIS with a pair of teleconverters as their long lens of choice. It's a back friendly size and weight with a nice price point with an excellent reach / brightness. One of them is considering swapping over to the RF 100-300/2.8 so they can ditch the need fro their EF 70-200/2.8 lens and just use the one lens. If Canon made a RF 400-600 f2.8 - F4 zoom lens that's a simular weight to my EF 400mm f2.8 II L and it's as sharp, then my ears might prick up and colour me curious.