I think it very much depends. There are players who come into the team and you KNOW they 'have it' i.e. Greenwood.
There are other players who come from slightly left-field and surprise people i.e. Rashford/McTominay.
However, you also have players like Brandon Williams who, realistically, really surprised everybody with his little purple patch. Even then, you couldn't fault the commitment and his effort, but at best I think you could describe his performances as 'rough and ready' or 'enthusiastic'. I never once looked at him and thought, "there's our LB for the next 5yrs".
In that scenario, loans don't work for me. Brandon COULD still continue to develop and make a good PL LB, however, I don't think he'll make that progress by going on various loans and never really feeling part of something properly. Likewise, he won't get the minutes he needs at OT.
Sell him, insert a buy-back. It's win-win for all parties.