A proven versatile defender that IS in his prime (he's 28 not 31/32) in fact a year younger than VvD and plays for a team that has far more similarities to our style of play (esp. high line) than most ... doesn't make sense? Whereas an unproven much younger CB is?
CBs hit prime in their late 20s, should we secure Alaba for next season that's exactly what he'll be ... in his prime.
TBC I have no idea if he is the defender we need, pretty much anyone is going to be a gamble (how many were/still are advocating Upamecano after he was ripped apart by United this week?) especially young CBs, but the facts are that he's a vastly experienced and versatile class player who would definitely strengthen the squad.