At the time Wanyama was certainly seen by all as a back up to Dier, don't recall anyone both professional or fan who said or expected otherwise.
With regards to the other signings you mention you forget about Davies who IMO is better in his position than Trippier. The others are usual collateral damage for any club. Name me one club that has signed players over the last 3 seasons that have a shorter list of players that haven't worked out, just ONE club. Not all players work out, you can't be expecting otherwise surely.
Everyone talks about how wonderful Cities attack is and it's the best in the League: Kane is better than Aguero, Son is better than Sane, Erikssen is better than Silva, Dele is better than De Bruyne, all of our boys have assisted and scored more than their counter-parts at Citeh. So I assume you can't surely be looking to buy a player that will start over our first team attacking unit. Our DM's are also arguably the best in the League, I personally believe Wanyama is better than Kante (he's got more to his game, better with the ball at his feet too), Dembele is World Class and totally unique (no player does what he does, there is no comparison with another player to make), Dier isn't as good as those two IMO, however his versatility also makes him a must pick, our defence is the best in the league nuff said, you can't be suggesting an upgrade on Rose, Walker, Vertonghen, Alderwiereld? Who are you going to buy and who are you looking to replace? Where is the weak link in our first eleven?