I completely agree with this, this season I thought we could get away with Fernandinho, Stones and Otamendi but that's not worked out as well as planned with Laporte's injury. It has just highlighted the need for a backup left sided CB and also a first choice CB to partner Laporte.
I think now would be an ideal time to go for de Ligt and Ake, give Stones an extra year simply because I don't think we'd get our money back on him anyway and it's worth the gamble if he matures.
The DM position is a nightmare one though because Rodri is first choice but we need someone who can partner him in certain games or at times within games, who is also good enough. Players good enough aren't necessarily going to want to come to sit on the bench.