Sure, I agree, but they know him, his attitude and his abilities. It's not a blind transfer. They expect to buy him. Of course, anything can happen. He could break his leg finishing his career. That's the chance with any loan deal and why it makes sense for the loaning club to insert a clause. We still don't know the clauses though but I wouldn't expect them to be difficult to be met. It's not beneficial to either party to make it difficult to buy or difficult to sell.Obviously that would depend on him not doing very well for them.
In that scenario keeping him out of the team and avoiding a large transfer fee with further contractual commitment makes perfect sense for them.