R. Elazar Moshe Horowitz (Pinsk) points out a very interesting paradox in the opinions regarding the mitzvah of limud ha'torah. R. Yishmael is the opinion who did not permit his nephew to study chochmas yevanis because the mitzvah to study Torah is incumbent on a person at ALL times, whereas R. Shimon Bar Yochai (quoted by R. Yochanan) says that one can fulfill their obligation through the reading of shema in the morning and evening. However, in the gemara Brachos 35b, it is R. Yishmael who permits working to compliment Torah study - הנהג בהן מנהג דרך ארץ, whereas R. Shimon Bar Yochai held that if one devotes himself to Torah the his work will be done by others. It comes out that R. Yishmael holds that one must learn ALWAYS, yet allows for one to do work, and R. Shimon Bar Yochai doesn't allow for work yet he holds that the mitzvah to study Torah can be fulfilled simply by reading shema?
Rav E.M. Horowitz suggests that according to R. Yishmael the demand is to learn every moment of the day, therefore one may not study other things. Yet, he allows for one to work since without the ability to earn a living one will not be able to focus on their learning - יפה ת"ת עם דרך ארץ. On the other hand, R. Shimon Bar Yochai held that there is no heter for one to stop learning in order to work, therefore the justification of those tzadikim who throughout history did work must have been based on the assumption that the mitzvah to study Torah is not at all times.
The approach within the opinion of R. Yishmael seems very plausible. The mitzvah to study torah is continuous and applies all day and night, therefore the mitzvah itself must include a dispensation for earning a living since otherwise it would be impossible to manage. However, his approach within the opinion of R. Shimon Bar Yochai is difficult. If indeed R. Shimon holds that the demand to study Torah can be fulfilled with shema, how can he put any restrictions at all on working?
It seems to me that R. Shimon Bar Yochai in Menachos is making a halachic statement regarding the fulfillment of the bare minimum requirement to study Torah. But, in Brachos he isn't making a halachic statement, he is rather recognizing a reality. If the Torah scholars would be mevatel their limud ha'torah for the purpose of earning a living, they may be permitted to do so but would still forfeit tremendous potential of achievements in their learning. R. Shimon wasn't making a halachic statement, rather he was saying that the Torah scholars shouldn't be mevatel their learning since תורה מה תהא עליה, therefore he was confident that to protect the achievements in Torah, G-d would make it so that their work will be done by others.