I don't think Wanyama was signed as a straight improvement on Dier. He gave us more options. Injuries to Alderweireld and Vertonghen also played a part. But it was also clear that Dier wasn't getting dropped at any point. Wanyama earned his place as the season progressed and stayed in. But I can't accept that he was signed at the expense of Dier. Never mind Winks as previously claimed.
Townsend was given a fair chance and plenty of game time. I think the players you mentioned were signed after it was clear he wouldn't make it. But he was given that chance. Same with Mason who was trusted and performed as a squad player up until the point out was decided he hadn't progressed sufficiently.
Onomah and Edwards haven't yet reached that level where they've been trusted with a real squad role (first or second choice). But under Pochettino that has happened with Kane, Bentaleb, Mason, Townsend, Dier, Alli and Winks. I imagine it's only a question of time before Onomah, Edwards, CCV or someone else joins that list. And when they do I expect they will fill a squad role that would otherwise have been filled by a new signing.