I think since this is a threat purely about texting, being able to text one handed is important to me since if I'm out and about moving, or maybe carrying a sixpack home or a bag of groceries, being able to text using one hand is important. That being said, if I'm at home on the couch or even just want to be able to text faster when I'm out and I can stop and still have two hands free, I prefer it.
I don't think the notion of "1 handed phone use" is a myth. But I think that even without the "reachability" feature, Apple has done away with needing to reach up to the top corners of applications (at least the default ones) in most scenarios, although certainly not all.