More than the quality (which you can always misjudge) it's deciding to buy the type of RB AWB is that puzzles me.
Even if we imagine he turned out as well as he could turn out, he was always going to be a defensive fullback who was limited technically. Which is an odd direction to shift your team in the long-run at a time when as much or more is demanded from fullbacks on the ball than ever before. When players like Trent, Robertson, Cancelo, James, etc. are the typical leading PL fullbacks it's an odd choice to opt for someone who even if he works out doesn't fit that typical profile.
It would be like scouting hundreds of goalkeepers when the time comes to replace De Gea only to buy another shot-stopper who is weak in terms of distribution, sweeping, etc. Even if they're ultimately very good at being the type of player they are, opting for that type of player suggests a lack of vision in terms of what the ideal side you're building towards needs to look like.
Because it was fairly predictable that if a new manager replaced Solskjaer at any point who did want those now-typical qualities from his fullbacks then AWB could be in trouble, even if he was playing well as he could as a defensive fullback. His poor form just meant it happened quicker than it might have otherwise. Hell even under Solskjaer we were already eyeing up Trippier in the summer.
I don't know if it was a lack of understanding of what was needed in the role on our part, a midjudgement of the type of player AWB was or a misbelief in the amount of technical improvement we could project ahead for him once he arrived here. Odd, odd investment of so much money.