Why did Hotch choose Morgan? Probably because that's what the script said to do.
On the other hand, Hotch know that Rossi is experienced, but may want to groom Morgan for a leadership role in the future, to teach him some of the restraint the team leader needs, which Hotch may not have so much of at the moment. Hotch may also be thinking that he may not survive his determination to free his family. We know he'll do whatever he thinks it takes regardless of personal danger.
I do not think Thomas Gibson has any plans to leave the show.