I’m more skeptical than you. I think the 5.4 will be almost impossible to use one-handed, though I really hope it is usable one-handed.I had the original iPhone all the way up to the 5S. When the 6 came out, I hated the size (the ability to use my phone one-handed and have my thumb *easily* reach every part of the screen is paramount. I got the original SE when it came out and still use it to this day. I'm delighted they are finally releasing something "small" but I'm still worried it might be too big given that it's...yes...only a little larger than the 5/5S/SE...but all screen which may still make it cumbersome for one-handed / thumb use. Well see!
Most people’s thumb can reach a 3.5” distance, somewhat fewer 4.0”. But like you say, 4.7 is too big for many to use one-handed, which is the reason they came up with the Reachability feature.
While the 5.4 won’t be quite as wide as the 4.7, I don’t think that will enable most thumbs to negotiate a 5.4 diagonal screen without changing grips, which is tricky to do one-handed.