If you want to hold land, you need to solve the prisoner’s dilemma problem faced by soldiers — namely problem of each individual soldier’s life being best if everybody EXCEPT him, does his job. Cultures usually solve the prisoner’s dilemma with elaborate ideas of duty, honor, selflessness and sacrifice, plus the punishment of those who violate these ideals.
I don’t know whether or not the cost of defense has gotten cheaper, or if it only seems that way. In any case, this video about Haredis in Israel is a good case study. Israel needs to hold land.