Maybe not LONG gone. Remember, McCoy was aboard in Episode 1. He might still be alive after just over 5 years, but he might not want to meet a still-young Scotty.

I don't think they had any way of knowing how long the sphere had been abandoned, at least not at the time of the episode. Maybe they found out later. It does seem odd that perhaps millions or even billions of inhabitants had all died somehow, but maybe the Borg found them or something.

True about McCoy. In fact I may be wrong about all of them except Kirk - he's the only one I know for sure is dead. They might all still be alive at the time when Relics takes place. As for the Dysonsphere it would be a major bummer to build something as massive as that only to have the star you built it around suddenly start putting out too much radiation or something. Data said how many M class planets it was equivalent to although I can't recall but it was a lot, so the number of people evacuating could have been enormous.

"Only to have" might be a bit of an exaggeration. The sphere could have existed and been populated for thousands of years before that occurred. Possibly even millions of years, which would mean that door technology etc really held up!

Hey, here's an idea! What if it was built by Losira's people?

I began a discussion (hopefully) about that on the TOS board.

And....?

It didn't seem to get much interest.