If Yoda and Obi-Wan didn’t know each other, how would Obi-Wan be able to tell Luke where to find him?
I said that them knowing each other was ‘convenient’ for the story/plot, not that none of the characters should know each other. Though, I wonder if it makes any difference whether Yoda was envisioned back then (when the OT was made) as having taught many Jedi, or just Ben/Obi-Wan and maybe also Annikin/Anakin (before he was ret-conned into having been Kenobi’s student)?
If we’re talking about story/plot ‘conundrums’ (the like of which you allude to above), this is in the same vein of making Leia the ‘other’ because, “otherwise, Lucas would have to bring in a totally new character that we’ve never met into the story”. Or the one about Lucas making Leia the sister only because “he couldn’t figure out how to resolve the romance of Luke/Leia or Han/Leia.” In the case of ESB, it was the conundrum of “who would take the place of Obi-Wan as Luke’s teacher”? He probably should have thought about that before he killed him off in the first one though, if you think about it. Conundrum aside, killing off Ben certainly made the first film work as a stand-alone.
The reductio-ad-absurdum of the Jedi as portrayed in SW and ESB is to illustrate why the ‘shrunken-universe’ criticism is misused when applied to Return of the Jedi (as a critique mostly used against Leia as the sister, but sometimes also against Vader being Luke’s father).