After our mishna says that Torah is fitting with Derech Eretz, it supports this statement with the reason that, "the effort of the two (together) makes one forget sin". The question is why do you need both Torah and Derech Eretz? Why is it not sufficient to put so much effort into either one or the other and also achieve the resutls of "forgetting sin". This is an especially acute question in our times, when a significant part of the men in the Torah world are occupied solely with Torah.
You can of course say that it is not possible to be always occupied with Torah alone. It is contrary to human nature to single mindedly devote oneself to any subject, even one as improtant and elevated as Torah study. For the times that one cannot study Torah, Derech Eretz is available to take its palce, and the effort of the both of them together, synchonously or sequentially, makes sin forgotten. Naftoli Hertz Vessely, early maskil that he was, in his commentary Ein Levanon says just that. However, how would those who argue for supremacy of Torah study over Derech Eretz explain this statement (except as posted before, translating Derech Eretz as "proper etiquette", which is difficult to describe as "effort").
In all fairness, there are many statements in Chazal that require that we interpret this mishna with care. On the verse in Mishle (5:19): “In her love (i.e. love of Torah) you shall be ravished always,” Rashi comments “For love (of Torah) you shall make yourself absent-minded and foolish, forsaking your own affairs and running to (hear) a word of halachah” (Eruvin 54b). In order to learn Torah, Rabbi Yohanan sold property which could have supported him in his old age (Shemot Rabba 47:5).
Rabbi Eliezer the Great, at the age of twenty-six, gave up a large inheritance and ran away from home to study Torah in Jerusalem at the yeshiva of Rabban Yohanan ben Zakkai, where he rose to unprecedented greatness (for an intepretation of this statement in the Torah im Derech Eretz tradition, see here).
However, it seems to me that our mishna follows a view which is already disputed by Tannaim. For a discussion of the disagreement between Tannaim on this question, see here. There are other examples.
The Gemara (in Ta’anit 21a) relates about Ilfa and Rabbi Yohanan who were suffering poverty and decided to leave the yeshiva and go out to work. After all, they said, earning one’s living is also a mitzva. They sat down to eat their lunch in a field, in the shade of a rickety wall. As they were eating Rabbi Yohanan heard two angels conversing. One said “Look at these two. They are leaving eternal life for the life of the moment. Let us push the wall over and do away with them.” The other replied, “No, leave them alone. One of them is destined for greatness.” Rabbi Yohanan said to himself, “Since I heard this and Ilfa didn’t, it must be meant for me.” Rabbi Yohanan went back to yeshiva, suffered, and eventually became Rosh Yeshiva and Gedol Hador. Ilfa went on to become a merchant, and unsuccesfully attempted to claim the position of Rosh Yeshiva.
Another example: Rabbi Meir states that a father is obligated to teach his son a trade; otherwise it is as if he taught him banditry. Later in the mishna Rabbi Nehorai (who according to Rambam is Rabbi Meir by another name) declares: “I will leave all trades and teach my son only Torah.” What about the obligation to teach one’s son a trade?
Some commentators pursue the solution along the lines of individualizing. They say that our mishna refers only to certain kind of people but there are also those who should enagage solely in Torah study. Pnei Yehoshua in Kiddushin answers that Rabbi Meir’s statement refers to ordinary people, while the son referred to in the second statement showed unusual alertness, smartness and desire for Torah-learning, so that he would certainly merit having his physical needs looked after by others. Alternatively, he showed at an early age the exceptional faith and trust in God needed to carry him through this type of life successfully (Sefer Hamakneh). The key is each one's nature and abilities.
Note the famous Rambam at the end of Hilchot Shemitta veYovel who also individualizes:
Not only the tribe of Levi but any person in the world whose spirit prompts him and whose mind convinces him to separate himself to stand before God in order to
serve Him... and who casts off the yoke of the many calculations pursued by other
people — he is sanctified, holy of holies, and God will be his portion in all eternity, and will also grant him a sufficiency for his needs in this world.
