These are great points I didn’t think of until now. Frasier’s best episodes revolved around the strong cast. Coming in 20 years later with a 99% new cast seems destined to not hold up well. The Crane Boy just doesn’t have the same ring to it.
I get that time marches on, but at a certain point, will the story actually reflect that, or will the disappearance of everyone simply be waved away with a line of dialogue? You’re right, the writers have an uphill battle on their hands.