You can already host games with any rank limitation you want, with the only rule that your own rank has to be within the interval.
In other words taking you as an example, the minimum rank cannot be larger than r6, and the maximum cannot be smaller than r6.
Premium is required for this though as it is part of the very old monetization system implemented 15 years ago.
The beginner-main lobby distinction is a workaround within this system to introduce an artificial barrier separating complete beginners from experienced players, avoiding an abysmal experience in which old players just destroy newbies.
It is not a perfect system but you're suggesting stuff on a "dead game" anyways. There's countless ways to implement it better but the owner has better things to do and he gave up on the game for now despite what he says (and I personally don't blame him for doing so, it's a sensible decision).
Also, this game has a golden rule regarding the staff that you're not aware of.
We do not talk about other games here. There is no war in Ba Sing Se.