"For my ally is the Force, and a powerful ally it is. Life creates it, makes it grow. Its energy surrounds us and binds us. Luminous beings are we, not this crude matter." You're mischaracterizing the subject to say that Yoda would actively seek such a power. Yoda was never actively seeking ways to become immortal, to do so is perverse, and the way of the Sith, not the Jedi. Qui-Gonn had obviously learned something during his travels and decided to pass on how to do so to Yoda at some point--well after his own death. Why can't both your highlighted points and the benevolence of becoming a Force Ghost be equally true in your mind? So everyone joins with the Force upon death, Qui-Gon figured out a way to commune with the living after joining with the force (and departing his "crude matter" body), it doesn't mean he did it to hold power over anyone or for any nefarious purpose.