Well, I think the intention of RWS is not bad but I think many of us simply won't accept the idea having just one rifle in the safe

I think the cartridge is well thought-out but for my taste, I would rather see its use when hunting heavy game such as moose, bear, buffalo etc. For me it would be much more interesting cartridge (and more versatile) than e.g. 458 Win Mag. I looks like shorter version of 400 H&H, which is by the way, my favorite DG caliber. Giving the construction of the cartridge, the belt is justified. The velocity and the power is very well comparable to 450-400 NE. See the link
https://rws-munition.de/fileadmin/datasheets/rws-10-3x68-mag-tm-bonded-25-9g.hunt.en.pdf
Here https://www.geartester.de/articles/3079 is an article about a hunter, who tested the cartridge and its effectiveness on the European game. He wrote - "Anything I shot at fell down immediately - walk away distance 0 meters"!