Serge is a free agent at the end of the season however since they are giving up a first round pick for him that tells me they are going to try to sign him to a new contract at the end of the season, you wouldn't "rent" him for the season and sacrifice a first round pick if you didn't plan on keeping him. He is a 3pt threat and none of the Raptors other power forwards are.
Ibaka is the prototype for what Siakam could become. Learning from Serge is absolutely good for Pascal, plus the Raps will go farther in the playoffs with Ibaka, providing some valuable exposure and potential minutes for Pascal.
Objectively, Toronto had kinda reached the limit of how good they could be this season with Siakam/Sullinger/PPat at the 4. Siakam is exceeding expectations for a WAC guy that was perceived a strong a first round reach last June, but he's not the starter on a Conference Finals team.