why we dont blow in a time of "tzara"

the rambam says that its a torah obligation to blow in a time of "tzara". the magen avrohom (o"c 576) asks why don't we blow today and fulfill our torah obligation especially since the rambam says that this mitzvah applies even in "g'vulim". the netiv Chaim there answerers that the rambam still means only in the holy land. the pri megdim elaborates that it is only an obligation when the "tzara" is on the whole tzibur not just one city and he also says only in Eretz Yisroel (i am not sure if he means that you need both most of klal yisroel + in eretz yisroel or not). What about today when we are all threatened by Arabs and nuclear weapons. What about 60 years ago? the netziv on the sifri (behaaloscha piska 18 page 237) says that the mitzva only applies when there is "lifnei hashem" which is referring to the Aron.