I think if most people are honest they would have thought the following, before we bought the below players:-
Sigurdsson 45m - Expensive, but a great player. Even the spurs manager said he would want him in his side
Schneiderlin 25m - Great at Southampton, better than we currently had, could be an unreal signing.
Walcott 20m - Better than we have, and in the current climate 20million is cheap for him.
Williams 12m - Had been brilliant for both Swansea and Wales, with pundits like Rio Ferdinand saying both Arsenal and United should target the player, especially at 12million.
Bolasie 24m - a lot of money at that time, but again gave us something different, and had a lot of potential.
Michael Keane 25m - Was a great signing, man u, liverpool both bid for him.
Tosun 27m - great goal scoring record, but an unknown who's been bought in a crazy market.
Sandro 7m - Spains best young forward!!! a steal
Gueye 7m - why do we want him!!!! both the Manager and DOF got slated for this signing, and has been a great success.
Klaasen 23m - ajax captain, dutch international... could be great.
Pickford 25m - overpriced, but could be great
I think at the time these players were bought, most of the reason for signing there players were justified. No one could had known that williams would had been as bad. Schneiderlin would have depleted so much as a player. Keane, would had been half the player he is. Sandro and Klassen wouldnt had been able to adapt.
You mentioned seri and the next word was "could" be amazing. Or he "could" be the next Klassen.Hindsight is a great thing!
Finding a Mahrez and Kante is a difficult thing todo, and if it was easy eveyone would be doing it.