The movie is building to such a crazy emotional pitch at that point that the lightsaber isn’t even really the biggest WTF. As soon as he shows up, you’re off-kilter in a dreamlike way. Did he shave and get a haircut? Did he de-age? And the conversation between him and Leia with the Luke & Leia theme playing is just so good that you don’t even really have time to think of what any of that really means, and then he winks at Threepio and I feel like even IF alarm bells are going off at that point, they get muted by Luke winking at Threepio and a lot of viewers just give up trying to tune into why this all seems weird and dreamlike, and roll with it. It doesn’t hurt that The Spark is so propulsive as a piece of music that you really are just firmly shoved into believing what you’re seeing and feeling no matter how odd it seems.
Was it predictable? Probably. Star Wars movies aren’t that difficult to figure out. There was really only the one time Star Wars really surprised anyone, and that was 1980, and that twist absolutely came out of nowhere both in concept and execution. But even if it was predictable, the way the movie presented it nullified that predictability. The scene had the punch it had whether you knew it was coming or not.