Muttley that's another area I want to highlight from my perspective:
Son usually hogs the LHS in the final 3rd though often runs in to cul-de-sacs. So, with Son being there, there's little demand for Davies to double up with him. Now on the other wing, Moura or Lamela have tendencies to cut in and take on defenders, thus leaving Trippier with acres of space. How many time have we seen a diagonal ball to Tripps totally unmarked thus affording him ample time to whip those crosses in.
Yes, I agree with you that Son offers less in defensive role (unlike Lamela) and tends to monopolise the LHS final 3rd.... may be that's why Davies doesn't advance so much or it's a case of knowing his weakness we rely on Son to do the deadly stuff up field. However even in Son's absence, we offer little threat from the LHS and that imo is down to Davies.