Generally speaking this summer has seen us linked with a lot of young players to replace some of our most experienced (De Gea, Herrera, Mata, Sanchez, Lukaku, etc.). Whoever we sign for RB would be taking the place of our most experienced player and probable club captain.
I know the caf's collective opinion is that anyone over the age of 27 is too old but really it's about balance. We need to have some experienced players in the side. If we're signing younger players everywhere else and we already have not one but two high-prospect youngsters we want to give chances to at RB then signing another youngster like AWB simply doesn't make sense for the profile of the squad. That's without going into any detail about flaws in AWB's own game.
I don't think Meunier is a particularly outstanding fullback but few are. He is good enough to act as a bridge between our current too-old and too-young fullbacks though. While being a good profile in terms of price, work rate, ability going forward, etc. A nice, easy, low risk signing that plugs a gap in the side.