I think most people don't really believe it but their cognitive dissonance makes them feel they simply don't understand it. I would not be surprised if such explanations are mostly made for borderline R&F who tend to question and scrutinize everything but are just too indoctrinated to do their own research. Most R&F are probably happy to be fulfilling Mat 24:14.
On topic, Jesus certainly was not vague about "this" generation. The importance of the generation has nothing to do with their overlap. If it did, then other examples would reflect something similar. If no other similar examples are found in the bible, then are there examples to the contrary? I believe there is.
We don't have to overextend on researching WT doctrine on the matter, but we are familiar with the parallels drawn between the deliverance of the Israelites and the one that JWs expect. The generation that left Egypt was made up of people of all ages. Likely several generations of people.
After failing to uphold their faith, in spite of having witnessed their rescue from Egypt, God condemns them by telling them they would be in the dessert for 40 years before ever entering the promise land. God had already tried to eliminate them all as he told Moises in Exodus 32:10 but Moises begged God to spare them. Wouldn't it make sense to let even the youngest of those generations die in the dessert of old age without seeing the promise fulfilled? So God did not burn them all in a second, but wasn't the outcome the same? In other words, none of the generations that witnessed the events of passover and such (Or where old enough to understand at least) seemed to have made it to the promise land. Witnessing, I believe is the keyword, not only on this accounts but in the case of Jesus' words as well as the bible as a whole. Witnessing and having proof of things has been a determining factor in many of the most significant events in the bible. Therefore I can not accept the overlapping generation teaching because the second crowd did not witness the "Events of 1914".
Sometimes I feel they are not even good liars. There are so many other things the could pin to their explanation to make it stand better ground but they are so fixated on this generation thing.