Consider that Anthem X 29er has a frame weight of just over 5 lbs, rivaling carbon frames, and you got their top level component spec for that alloy frame. What did it come in at, 4k? That's an XC bike and at 27 lbs for an XC bike, for anyone riding true XC, that is high enough that you would want to put it on a diet.
With 29ers, you might be able to save weight in a few ways. Downsizing your suspension travel needs and your tire beefiness needs are two rather major options. I never fail to see people, claiming to be "trail riders", riding 26" FS bikes with way more travel than they actually need, maybe with 140mm travel bikes (ex. Giant Reign) on a XC trail. 29er riders seem to be saying that they can do almost all the trail on their XC FS bikes with 100mm travel, but they're not really willing to do drops and jumps like they would on a slightly longer travel bike, and hesitate to take it to the lift-served shuttle trails, when they really are interested in such riding, as the frame and suspension doesn't seem to be made stout enough for such. It's not that uncommon to see dropper posts on 100mm FS bikes around my area. Giant is now offering that in-betweener 120mm 650b bike, that might actually do it all. 27 lbs is actually a decent weight if you're a trail rider that wants something trail-tough, and having the frame, fork, and wheels (and other stock spec) being designed specifically for trail, rather than XC, would help cut expenses.
Not too long ago, 29er riders were demanding stiffer forks. Enter the Fox 34 and 15QR. Fox 34 turned out to be too heavy, and some consider the 15QR to be not enough, wanting the 20mm axle and the bigger hubs that come with it. People were also complaining about 140mm 26" forks being too flexy. Well, now 32mm forks got stiffened up, with an improved stiffness to weight ratio and then there's the new Pike people are raving about. That fork is expensive at $1k. There's a lot of options at $400 or so, but they're not too well suited for the kind of bike people seem to want, which is more than XC, but not too much that it's more for the DH/shuttle/lift crowd. A 120mm 26" might seem like not enough, while a 120mm 29er might seem flexy... 120mm 650b seems like an interesting compromise, that might save you from the expense of going with a Pike and building a bike around such a fork, which actually might be more than you actually need. Reading up on reasons to buy the Pike 29, it seems people wanted something stiff (stiffer than 32mm forks), yet lighter than the Fox 34, and since the A-C measurement was lower than the Fox 34, people were opting for more travel as well... a 650b 32mm fork might be sufficiently stiff and lighter, without all of that [excess] travel that could lead to inefficiency for general trail riding.
The more I check out 650b stuff, the more it seems to make sense, *especially* now that I see that it combines all the 29er-driven innovation and proven concepts from both wheel sizes, to please the demands of riders these days. It doesn't seem to be backwards or anything. People have warmed up to the advantages of a bigger wheel size with the 29er, but they are demanding that it be improved in ways that are challenging. I think 650b is one of the answers, if you're one of those riders demanding such. Probably not for everyone, but I'm starting to think trail riders will find it appealing. I'm starting to get "sold", at least looking at it objectively from the problem solving point of view. This 650b movement seems to be an answer to that silliness, compromising on travel amounts, going too XC or too AM than suited for your style, on paper at least. For me to be sold, it's got to prove it to me on the trails.