I think it all has to do with how things transpired throughout a band's career to get to the point of having one, or even no (which has happened) original members. I think if a band had gone through a shitload of changes before releasing it's "essential" material, no one really cares about the lineup changes before that, just what happens after. Take a band like Maiden, who went through countless changes through their pre-debut years and even the first 3 albums. But Maiden released it's "essential" material with a stable and consistent line up. To most people, that's Maiden.
If a band has been around 20+ years, it's common that they can wind up with just one original member without it being weird at all. If people just come and go casually every now and then, it just seems normal as it happens through the years.
Also, for a lot of people, as long as a band retains it's most popular singer, or if there is a primary writer, that's what defines the band for fans, and as long as the stay, people don't get that upset with the changes. For example, Iced Earth. It's been Schaffer's show from Day 1, and people know that. People are biased towards singers (usually Barlow vs. Ripper), but there isn't anyone out there who think the current lineup is blasphemous because of the other new faces.