Alderweireld is an excellent player but every club has a wage structure. Tottenham don't have the same budget as City or Utd - the only 2 teams above them.
Aurier is a decent back up full back. What hurt them against Juventus was Ben Davies, who is decent but not as good as Rose.
OK, never mind positions 7-20 - here's a list of Top 6 CBs that Sanchez is better than: Stones, Lovren, Smalling, Jones, Rojo, Mustafa, Koscielny. Is he better than Duffy? Yes, although Duffy himself is better than a few of those I've listed too.
As for buying duff CBs, they normally get...
1. Sanchez is an excellent player who would walk into most of the other top 6 sides and everyone below them.
2. Alderweireld is in a contract dispute with Spurs and currently injured.
3. Aurier switches from right back to left back for Rose, then promptly scores.
Do you know anything?