Even at launch, I do not believe you had to play multiplayer to get a sufficiently high readiness rating to see any endings. There were other tie-in like the Datapad app that contributed. The expansions and Legendary Edition re-release made a lot of changes that lowered the readiness requirements and totally removed multiplayer anyway.
You did miss out not playing the multiplayer mode in my opinion. It was very fun!
You did miss out not playing the multiplayer mode in my opinion. It was very fun!