As Lucas always intended the ST be made this decade (per a quote from Mark Hammil about episode 7 coming out in 2011), that means a long gap between trilogies so holding the revelation of “the other” until the ST makes no sense. He got the fans buzzing after Empire so it makes sense his plan was always to reveal the other in Jedi. Why make them wait for 30 years.
This (like many things you’re saying) is simply not true. Lucas’s original intent was well, a lot of different things when it came to the sequels, but basically a 12 episode saga, each directed by different people except the first and last. This morphed and fluctuated with the 9 episode saga and such. The other was definitely supposed to be the subject of episode 7. Lucas decided while developing ROTJ that he was tired of the saga and wanted to tie it up (though knew he’d get back to it later).
The idea of Luke as an Obi-Wan 30 years later wasn’t decided until after ROTJ was developed. Notice that Hamill’s remarks about episode 7 in 2011 is from an interview in 1984.