You know a lot more about those players than I do, but I suspect that's typical fan naive optimism . It's rare that players make the step up, and we see it a lot from United fans. A few years back, it was Cleverley, Welbeck, etc and now we realize our problem was that we were plugging holes with players who probably weren't good enough. People have said it this season, listing players like Zaha, Powell, Keane, Varela, Lingard and it's not often that one can make the step up.
Granted, I know that you are talking about squad fillers - players who are competent enough to step up when (if) they have to, but depth in numbers and depth in quality are two different things. I would be very surprised if any of those players become stars - though I am interested in what Suso can do.
Your point about your main defenders being out but you've coped - Toure alone cost you 2 points against West Brom and almost more again against Fulham. And what's to say you won't have similar bad luck next year? Us United fans bemoan our injuries to centre-backs every year and have been for a long time. In recent years we had Ferdinand, Vidic, Evans, Smalling and Jones - 5 competent centre-backs and still every year struggle at some point.
The league will once again be tougher next season, with every team strengthening. You would expect Spurs and United to both do better next season, and Liverpool will have a ton more games to worry about. I don't think 3 signings will be enough to keep you in the top 4 next season.