The question has been raised in contemporary times vis-a-vis Israeli soldiers. (And if I recall correctly from a lecture by Rabbi J D Bleich, Jewish soldiers in the British Army during WWII as well.) As Rabbi Bleich pointed out, in today's information era there are exceedingly few cases of people who outright disappear at war. (To prove his point, he asked "how many US Vietnam MIAs are there today?") An additional wrinkle is that the Get would say "if I don't return within time X"; often with today's wars it's not uncommon for soldiers to return home every so often, not quite knowing when they'll be deployed; so a soldier would have to do a new retroactive Get each time he leaves.
The retroactive divorce was also proposed as a solution to the other agunah problem ("Type II Agunah", as Rabbi Breitowitz puts it; or as Rabbi Rakeffet says, "man-made agunah"), that of the husband who is clearly alive and right here, but refuses to give a Get. Why not, the day after the wedding, have the husband hand his wife a paper that says "you are hereby divorced from me right now if at any point we spend 2 years not living under the same roof."
The objection to this came in the early 1900s from (if I recall correctly) Rabbi Isser Zalman Meltzer, who raised the following possibility of an unintended consequence:
In our current system, there are plenty of guys out there who are going through rough times in their marriage, but will stick it out and try to improve things. (Or if G-d forbid things really are serious and irreparable, go through the right process to make clear it's over and how to settle it.) They know that leaving a woman an agunah is utterly reprehensible.
But if we have everyone giving their wives this conditional divorce now,
what's to stop a husband from running off to who-knows-where and abandoning his wife? He knows that in 2 years no matter what, she'll be free to remarry!