Luke builds a new saber after the events of ESB and plans to return to Dagobah to train with Yoda.
So if it was just a matter of Yoda losing his, why didn't he build another one?
If he needed one in the PT he would be more in need of one in the OT (to defend himself and to help train his eventual apprentice).
If Dagobah's atmosphere was too funky to use a lightsaber why did Luke use one in ESB?
Anyway you look at it Yoda in ESB is a totally different, more powerful and more interesting character than Yoda in the PT.
Would Lucas have made him as stupid a character in the OT if he could at the time?
I don't know?
Would he have given Palpatine a saber in ROTJ if he had thought about the potential for toys and such?
*shrugs*
The bottom line is those characters are more powerful and more interesting without sabers than with and Lucas screwed up what was there with what he latter added.
It's like giving Duke Dooku Palpatine's Force Lightning.
It weakened Palpatine in ROTJ by giving his underling the same 'secret power' in an earlier episode.