In my humble opinion, etc.
There has been a trend towards more travel as years go by, I think this has been driven by demand, rather than necessity.
In other words; we like the idea of more travel, so we've asked for / bought bikes with more travel, whether we've needed it or not.
we've needed a lot of technology to make long-travel bikes work - they're crap and un-pedally without clever shocks and pivot placement.
29ers, and 650ers, have forced/allowed designers to use all that technology on different platforms, and at the same time, half-ignore the accepted wisdom regarding travel and application (100mm = xc / 120 = trail / 150mm = all mountain, etc.).
It turns out that modern bikes are brilliant, not because they've got more travel, but because they've got better designs. A 120mm 29er might feel almost as capable as a 150mm 26er, not because of the wheel size doing magic things, but because they're designed to do the same thing.
in other words, we'll accept the idea of a 140mm 'All Mountain' 29er, but we wouldn't accept a 140mm AM 26er - we'd expect that would have to be 160mm surely?
(numbers more or less plucked out of the air, but i hope you get the idea)
and yes, i do suggest that 26ers could be designed with less travel than we'd expect for their application, and mostly, we wouldn't notice as big a difference as you might think - design is much more important than just the amount of wheel travel.
so, a 100mm 29er might actually feel similar to a 100mm 26er, or not. mostly it'll depend on how they're designed to feel, rather than anything intrinsically linked to the wheel size.