First of all, it is important to realize that there are very few sovereign countries that engage in terrorism as an instrument of policy. It may be that Iran engages in it through proxies, but there are few other countries that do so. Terrorism is more of a tool of smaller groups such as Al Qaeda, not of states.
Terrorism has some appealing attributes as an instrument of war because it allows much weaker groups or countries to strike at much stronger ones. Al Qaeda is a perfect example of this. There is no way that this group could ever do any sort of serious damage against the United States in a “real” war. It simply does not have a large enough membership or the sort of money that would be needed to field a formal military that is strong enough to harm the United States. The same goes for many countries of the world. Terrorism becomes very appealing because it allows the terrorists to accomplish something that they could not accomplish through any conventional means.
As for whether terrorism will become “the accepted method” of warfare, this depends on one’s definitions. Terrorism has long been the method of warfare that is most accepted by weak groups as a way of fighting strong groups. In that sense, it already is “the accepted method” of warfare for the weak. However, if we are talking about the opinions of the world as a whole (not those of the weak groups), terrorism is not likely to become morally acceptable. Terrorism causes too many casualties among innocent populations for it to become morally acceptable to the vast majority of people.
Thus, terrorism will remain popular and accepted among weak groups and some weak countries because it allows the weak to hurt the strong. But the dubious morality of purposely attacking innocent victims will prevent terrorism from becoming morally acceptable to the majority of people in the world.