All clubs will have examples of players they could and should have signed.
Bruno Fernandes is one example. I don't think it was ever an option to sign him, GLC and Ndombele. Particularly without selling Eriksen. So we needed a decision, the decision seems to have been GLC and Ndombele. If Pochettino was against that, argued for Fernandes instead, fair enough. I don't see how you know that.
We should have also signed X player type arguments doesn't really do it for me.
Part of the job for the manager and recruitment department is to identify players that will work even if your first choice isn't possible for whatever reason.
I wanted Wijnaldum, but got Sissoko type arguments similarly doesn't do it for me. Why was Sissoko on the list in the first place? Why didn't the manager veto it? Why weren't there more players like Wijnaldum on the list?
Hey, perhaps Mourinho just got super lucky and all the players he wanted were his first choice players for those positions. I somehow doubt it. Hojbjerg was a great buy, in part because we were able to identify a player we could actually sign. If he went to Liverpool it would have been up to Mourinho and the recruitment department to identify a similar player, several similar players, players we could sign.
I'm sure there are players we could have gotten over the line. Part of the job of the manager and recruitment department is identifying those players we should push the boat out on, over and above other players we are "interested in". Perhaps Pochettino did that with Grealish and Levy just stubbornly didn't listen. No way to know.
Regardless the job is then to identify other players that fit in, that solves problems rather than becomes one. It's difficult, being good in these areas means outperforming others, not being perfect.
Pochettino started very well in the transfer market, at a time when we could gamble on cheaper players and it didn't take that much to improve on players in our starting 11 because we weren't very good. As the job became spending more money, getting real quality in to make a quick impact on an already good team he didn't do as well. It was difficult, it is difficult, but he didn't do particularly well.
It left us in a bit of a mess sadly.