We know that Arda Güler complains that he doesn't get enough minutes. Especially in the Turkish press, there is a lot of news about this issue. I think he will leave Real Madrid at the end of the season. It would be best for his career if he leaves on loan and goes to the Premier League. Arda Güler is currently a side character at Real Madrid as a substitute player. At Bournemouth he can find enough minutes and become the main character. He has the potential to do that. He is only 20 years old. But he doesn't like being a substitute. I think he is a boy with a leader spirit. He can improve himself at Bournemouth. And he can move up a level.
Real Madrid has too many good players played in Arda's position, and makes him unable to compete. It's why i think let him leave as a loan is best decision. He gets a lot of advantage, and his club is also able in signing a new back to strengthen their squad.
I don't see negativity in this case. It's all good for his future, and he clearly must get more minutes in order to develop himself better. It's better rather than see him warmed bench, and has no use to the club.
Just as you have said Arda Guìeŕ situation at Real Madrid is indeed challenging given the stiff competition for this position, with top player vying for the same spot well if you check it is to tough for him to get sufficient playing time, a loan could be beneficial for hims growth allowing him to gain more experience and minute on the pitch.
To me his development is crucial at this stage in his career and a loan move could be perfect solution this would enabled him to play regularly and also gain some confidence and eventually return to Real Madrid as a stronger, and more experience player.