“Mowing the lawn” would have just allowed the grass to grow another day, except that grass is harmless but rocket launchers are not.
The Talmud – and history – have generally looked kindly at Yoav overall, whose decision in the instance discussed above was ultimately deemed justified (though there is a lot of nuance that may not necessarily be fully understood or agreed upon). Later on in the same page (49), Rabbi Abba bar Kahana opined, Were it not for [King] David, who was engaged in Torah learning, Yoav would not have been successful in battle, and were it not for Yoav’s skills as a warrior, [King] David would not have been able to study the Torah. And, we might add, might not have been able to produce so much Torah through his classic writings that became an essential part of Tanach.
Of course this discussion goes far beyond Yoav and King David, and is more relevant now than ever, as we contemplate the importance and essential role of those who study the Torah in Israel and the important and essential role of the military, though arguments can be made on both sides – King David was able to excel in both areas. But to paraphrase Senator Lloyd Benson addressing Senator Dan Quayle in the great debate of 1988 (You, sir, are no Jack Kennedy): Not many of us (none, actually) can compare to King David, though many may strive to emulate him, each in his/her own way(s).