Adding an expiration adds some extra weight on the chain that I don't think is really necessary. I assume the majority of the use cases for this are going to be automated and worst case it's fine if you have an outgoing delegation that you forgot, it's not like you'll miss the rc since you use like a fraction of your total RC and it's not like hive power where if it's delegated out you lose out on curation rewards.

Plus I assume there will be a neat UX to show you a list of outgoing rc delegations.