I don't understand why that's not okay, though. I get people get attached to communities they're a part of, but just like people, they change and/or die.

If you think about it, it's absurd to expect them not to: Even if the community keeps its member list exactly intact forever, and the members remain at their exact same activity level forever, those very members will change over time. Hell, you will change over time and get to appreciate different qualities of that community.

Same reason my guild's forum feels completely different today than it did 5, 10 years ago.