My t'penneth worth
Of course, it depends on what Apple actually deliver. If, as some rumours have suggested, gen 5 delivers the power of gen 4 and comes in significantly lighter (without necessarily being much smaller, length/width wise) then I think Apple will have a winner.
I think a big problem Apple have had with gen 3 and gen 4 is that the iPad 2 is so good, it's still very relevant. Look at the damned thing. It still performs slickly and pretty much flawlessly with the latest OS upgrades. It's as much as a lot of owners need, so I think it will the physical aspects/ergonomics of a new device that have the best chance of getting existing iPad 2, 3 and gen 4 owners to part with cash.