Apparently the loan rule where you can't play against your parent club is set to be scrapped. I think the SPFL still need to ratify the rule which will probably see it not happen but Uefa and Fifa have been working to ensure this isn't a rule. For instance in champions league it is not a rule. Edouard and Roberts in the past have played against there parent clubs. They have also scrapped the cup tied rule recently for the latter stages of the european competitions.
Were that to be the case I wonder if the end result would just be teams stopping loaning players to others in the same league? Obviously cited examples exist of teams loaning players to other teams they have played in the Champions League, though there is an element of the parent club taking the chance they won't be drawn against the team they are loaning to. That dynamic is obviously different in domestic football.
On balance, and bearing in mind the points that not every loanee will successfully return to their parent club, I wonder would we be better off having had the benefit of someone like Christie / Morgan for the 18 months even without being able to play against Celtic (and the unknown of a cup final winning goal in the future) rather than not having their qualities in the team at all. Arguments abound about the need to develop talent, but short-term ism and results based business carry a lot of weight.