If he brings in another 4-5 of his own signings this summer and we miss out on Europe again next season and finish 8th-12th with no trophies at that point would you question if he is the man to take us forward?
it depends on the situation at the time. two bad seasons in a wrong is certainly a concern and there wouldn't be many other managers that would last that long. Even Wolves had one bad season this year and he's out.
That said, I doubt a real solid foundation can be fixed over one year, that's my real concern. The two times its happened Chelsea and Man City, took insane amounts of money to get legitimate improvement. Many teams gain a bit of improvement, until their weaknesses get exploited and they regress again. If it was so easy, other teams who've spent money could jump the queue as well.
The issue for Everton is that they need 8-10 legimate good players this summer especially if they're going to to get rid of 4 or 5 players (hey maybe more), Siggy, Gomes, Bernard, Iwobi just to name a few, because we had a lot of games where the entire bench was just academy players. I just don't think that's happening.
Honestly who's going to make Man City's team from the Everton team - not a single player... peak Richarlison maybe, and he's behind all those players playing his position. Richarlison needs to have a good long look at himself honestly - he's behind De Bruyne, Foden, Mahrez, Sterling, Torres.