Sounds like the location was a last minute afterthought once they abandoned the underscreen button. Terrible location but I guess it will be a one year only feature if they push foward with the underscreen button on S9... possibly in time for Note 8???
consider this... the S8 was to have underscreen button AND dual cameras... and design and internal hardware layout pretty much near production stage. They realize they can't get the bugs out of the underscreen button and have to wait until S9 and now they have to design a button in the back but everything has been designed and would have to redesign the internal layout! Viola... remove the dual cameras, put one camera back in and use that freed up space for the power button.
A lousy place to put a button... but problem solved. :/
The only thing I don't like about it is the absolutely horrible placement of the fingerprint scanner. This phone does so much right, but that one failure could kill it.