FYI... All the new bike frames have less than 69 ° slack and short chainstays. That frame would be obsolete.
That's a bit of a BS statement. Go look at Niner, who focuses on 29er builds. Their hard tails are 70 degree and just redesigned this past year. Also the Giant XTC Advanced sports a 70 degree. If memory serves me correctly, the Specialized Epic hard tail is > 71.
Obsolete, absolutely not. Granted the chain stays are a few MM longer depending on what bike you compare it to, but a lot of the bikes handling has a lot to do with the complete geometry, not just head angle and chain stays. Then you have the fork itself. What is the offset? This will also come into play with how the bike handles.
Some things to consider:
Where will you be doing most of your riding? What will your average speeds be? I was in a similar situation as you, but I came from a 26" wheel. I purchased the CarbonSpeed MB-01 and I did so based on the geometry, and being located in southwest Florida where it's generally flat with a few rock quarries around to get a few drops and technical riding in. Basically, my average speeds are between 10-16 mph most of the time. Having a BMX background, I wanted to build an extremely agile 29er, which I did. I actually pushed the boundaries by doing 2 things:
1 - I went with the Fox Factory SC fork in 44mm rake instead of 51mm
2 - I'm 5'10" and on the border between a medium and large frame. I went with medium to shorten the wheelbase just a little.
Coming from a smaller bike, something else to consider is your wheel setup. You will want to minimize the rotational weight was much as possible, because when a wheel is spinning, the gyro effect fights changing direction. I would recommend carbon rims, tubeless tires, and try and select tires that are < 700g.
