The story goes is that Gilligan's Island was canceled (https://en.wikipedia.org/wiki/Gilligan%27s_Island#Cancellation) after only three seasons in part because the wife of CBS head William S. Paley didn't want Gunsmoke to be cancelled at the expense of giving GI a fourth season (https://tvtropes.org/pmwiki/pmwiki.php/ScrewedByTheNetwork/LiveActionTV). The easy answer would be to say that it would be like the Rescue from Gilligan's Island (https://en.wikipedia.org/wiki/Rescue_from_Gilligan%27s_Island) TV movie that was made 11 years later (sans Tina Louise).

They probably didn't plan on ever doing a finale. I don't think shows did big finales back in the '60s. I mean in 1968, they knew The Andy Griffith Show was ending and yet they didn't give it some big send off. I guess the short-lived Hank got a final episode, so it wasn't out of the realm to do a big finale, just not as common. I had always heard the reason was because it negatively affects the reruns. That's something I never understood, but after The Fugitive had its' big series finale, it did poorly in reruns. So maybe since Hank only had a season, and they figured it won't be rerun a lot anyway, they may as well give it some proper closure for the current fans.

But getting the castaways off the island would have been the best ending they could have done. That way it's the first episode where they arrive, the series where they're on the island, and the final episode where they finally return home.

They probably didn't plan on ever doing a finale. I don't think shows did big finales back in the '60s.

So you're implying that, with the premise it had, Gilligan's Island must have aired in the wrong era, and if Gilligan's Island aired in, like, the 80s or later, they would have easily done a finale episode. GI ending on a regular episode (which it actually did) would ruin the show for modern-day viewers. The castaways would probably be stuck on that island for literally decades.

Did Gilligan's Island really air in the wrong era? And was its premise really terrible?

I think it's safe to say they would have done a finale if they had the chance. It's a show that's just begging for closure. I always figured the reason finales were so hard to come by in the 50s and 60s is the networks canceling the shows without notifying the producers. But then you have Leave it to Beaver and Dick Van Dyke, which were ended by the creative team itself rather than the network, and both had explicitly final episodes.

Did Gilligan's Island really air in the wrong era? And was its premise really terrible?

I honestly couldn't see it airing in any decade other than the 60s. It seems to perfectly embody the spirit of sitcoms in that era. And in the big picture the lack of a finale isn't much of a deal breaker.