Michael Hann at the Graun on bands still touring and recording but with no original members left - among them Molly Hatchet, Napalm Death, Soft Machine, Yes (debatable but strictly-speaking correct) and Odyssey.
The current incarnation of Gong is another example.
It's a bit like organ replacement and limb transplants taken to the extreme, such that there's no original body left.
Perhaps it's not so odd... You could see a band as an institution (like a school or hospital) or a commercial entity (a shop, a company). Why shouldn't it perpetuate itself after the resignation, retirement, or decease of the founders and original staff?
Then again, in rock and pop, so much of what makes a band is the players as individual instrumental voices and the almost-instantly-identifiable composite of their playing as the band's sound.
Likewise the voice of the singer - and in some cases how the lead voice meshes with the harmonies of other members.
(E.g. the way Fleetwood Mac got increasingly ersatz in certain phases when most of the principals had left).
(Conversely, you could take Fleetwood Mac's journey - with original central figure Peter Green long gone, the arrival of a wholly new creative engine in Buckingham-Nicks - as a good example of evolving longevity in a band-as-institution.... That said, they always had the continuity of the rhythm section, which was also the source of the band's name)
Given that rock and pop are audio-visual hybrid forms, you could further say that the physical appearance - the face, body, expressions, gestures, mannerisms - of the performers are part of what makes a band a band.
In fact, given that rock and pop are audio-visual-verbal-textual hybrids, you could further-further argue that a particular way with words, an imagination, the personality and humour of a band - expressed in lyrics but also interviews, stage banter - is what makes a band a band. (Oasis isn't just the songs, the recordings and the concerts, it's the interviews, the ongoing saga of Liam versus Noel... )
In rock and pop, you are consuming audio-visual products but you are also consuming identities.
If a band is both the personalities in it and the collective personality of the group, how can that still be said to exist in the absence of all the original persons that constituted its make-up?
In the arc of ersatz, at what point does a band where all the originating members have left effectively morph into a tribute band?
Some members are more integral and identity-constitutive than others. Ergo, they should have disbanded after Moon died... like Zep did after Bonham.
