The problem with the idea of a radically different Push 3 is it throws away years of development and feedback cycles invested in Push 1 and 2 (which are fundamentally the same, other than the much nicer display). People have spent years learning to use Push as the centerpiece of production in the studio and/or building up live performance setups around it. Major Ableton Live updates come with significant additions and improvements to Push, including Push 1 (and they say
"We plan to continue supporting Push 1 and to adapt our new features to it, whenever possible.").
To me it's clear Ableton is committed to the existing Push controller and sees lots of potential to keep improving it. A total redesign would throw out all that progress and make them start over from scratch, which — as anyone who's used Ableton Live for a long time can tell you (it still looks and works pretty much the same way it did in version 1) — is not the way they do things.
Since a total redesign doesn't make any sense, whatever improvements they'd like to make to the hardware (I'm sure they have ideas) would have to be incremental and not disrupt anyone's established workflow. My point being: Even if they're working on Push 3 right now and it happens to come out a week after you buy a Push 2, you still have a fantastic controller that's built specifically for the software you use, and if the past several years are any indication, it will continue to be supported and improved for many years to come. And if you really must have some new feature in Push 3, well, maybe they'll have a trade-in program like they did when Push 2 was released (you could send in your Push 1 for 30% off a Push 2).