What is the point of having this teaching then? If all of those who partake are going to heaven anyways?
Right, OTWO. The point is, too, that they want to keep the 1914 year as the date when Jesus started ruling in Heaven, and then 1919 chose the Faithful and "Discrete" Slave. The overlapping generation allows them to keep teaching they have authority over the flock as the one and only channel to God--teaching as they have for a century that Armageddon is coming within a generation.
Take away the overlapping generation teaching and the immediacy of Armageddon coming (within a generation of 1914) falls apart and it becomes clear their doctrine is just a bunch of crap.