If Walker is so fabulous and Trippier so bad, surely Poch wouldn't have allowed him to leave like he didn't with Dier and Rose, we had him on a newish contract.
And again, if Walker was deemed so superior, Southgate had no such issues as you say Poch had, he would have played Walker as the RB and just put one of the other CB's (like Dier) into the RCB role.
And you thinking Aurier is better than Trippier, isn't answering why you think after 12 months, and paying 25m for him, Poch is still preferring Trippier even more now than 12 months ago.
Managers don't get everything right, just because Poch and Southgate prefer him doesn't conclusively prove Trippier is the better package, I don't think it's black or white either, but I think there's pretty solid anecdotal evidence, supported by solid statistical evidence, that Trippier is the better player in the right set up at least.
Either way, having the most creative and productive right back in the PL is hardly a catastrophe, even if he was a complete disaster as a defender, but he isn't, at worst it's a tactical issue that should be easily solved by either playing a back 3, Wanyama recovering or playing a more dynamic CM set up. The Watford game was a pretty good example of part of that, as the Statsbomb pice by James Yorke pointed out.
And either way, midfield where we have (a CB) Dier who can't play under pressure, a knackered Dembele, fucked Wanyama, Fucked Winks (who's progressive but also has defensive issues), Sissoko who's got the technical finesse of a Rhino on PCP and two No.10's filling in, is a much, much bigger issue.