Don't know about this other guy and Ryukyu, I just looked at the Mongol screenshots.
In my view, WC should be virtually impossible to achieve, other than for extremely good players (requiring some luck), or even totally impossible. And some countries should be even more difficult than that. Unfortunately this is not EU4 at the moment.
You should probably get familiar with aformentioned 'extremely good players'. DDRJake is one of the best players in the game and the first to achieve Three Mountains achievement with Ryukyu, Paradox has dedicated multiple patches to closing his exploits. TMIT is also probably one of the most dedicated Horde players in the game.
The problem, I believe, lies in that every player expects to conquer and expand, independent of their actual skills, and independent of the country they're playing.
You should try to get familiar with what is going on instead of blabbing out uninformed assumptions.