Right now, the 587/607 is a major weak link; doctrines can be argued in circles for days, but the overwhelming evidence for 587 is damning to their entire theology, and it can be found with very little effort.
However, if they just bite the bullet and "new light" a 587 date for Jerusalem's destruction it solves a lot of their problems:
-Removes their main weak secular point of argument
-Pushes the start of the Kingdom to 1935, giving them another 20 years
-Lets "woe for Earth" make more sense: September 15, 1935 The Nuremberg Laws go into effect in Germany and other indications of Hitler's slow rise to power, vesus Satan being cast down to WWI already in progress.
-Lets them completely throw Russell under the bus, so it's harder to dispute being chosen by Jesus for their early teachings
-Would actually lend some foundational reason to say why the Earthly hope suddenly came up in 1935
Sure, they'd lose a few, but they're losing them now anyway. Not much of a long-term downside to it; I bet they'll have to do this eventually.