I think I read where these PvE missions will be permanently available in future seasons. The problem is that by the time the new missions come out (probably in a few seasons), no one will be queuing for the invasion missions.
This presents a great opportunity for Blizzard to put together some extra challenges for players wanting to solo these old missions.
I know if a player quits mid-mission now a bot takes over. Great. Is there anyway to queue for a mission by yourself now?