Guessing how younger players will turn out is very difficult since there are so many factors involved.
If a player is at talent level of a Trent, Sterling, Owen, Fowler then it's easy to predict that they will have a very good chance to make it long term. In our current group of young players I think Elliot is the only plater who potentially gets into this type of bracket.
With all the other young players we have, from Grujic/Wilson through to van den Berg/Cain, it's much harder to predict how they turn out. 2 or 3 could easily end up playing +100 games for the club (which I'd view as the very top end of the likely success scale) or they might not make 100 starts between them (nearer bottom end of scale).
You've only got to look at the past to see how difficult these things are to predict. Suso, Nemeth, Insua, Flanagan, Kelly, Stephen Wright, Pacheco, Ibe, Kent, Solanke, Rossiter, Welsh, Coady are just a handful of names over the last 20 years that at some point people thought would have a good chance to make it in the Liverpool 1st team. None of them really did for a sustained period. Though they have gone on to have varying degrees of success in their respective careers.
I think the likes of Hoever, Jones, Brewster, van den Berg, Kelleher, Grujic, Wilson, Woodburn Larouci et al are in that same bracket where it's hard to see their ceiling. Maybe less so for Grujic and Wilson given their current experiences but will they end up playing for Liverpool regularly ? Or is their ceiling being a very good Premier League player at a mid-table club? Or is it a career in the lower leagues? Really hard to guess for anyone, never mind supporters who don't see them regularly or in training with the 1st team squad.