Agreed about The Who. But they did it anyway, and they've still been out there selling out huge venues. Arguably they're in an even worse situation, with only 50% of the band still being alive!
As for Zep, I thought the 2007 Celebration Day reunion gig was great - Jason smashed it, and as a two-nights-only thing, it was enough to cement their legacy, plus we got a good 4K Blu-Ray out of it, for all of us who wore out our VHS copies of The Song Remains The Same!
You could apply this to any number of other bands with dead/retired members - I hear it a lot about Queen, for instance. I feel the same way about them that I feel about this Rush tour - people want to hear this music, preferably with the involvement of as many of the original musicians as possible. And that's what they get. I have no issue with Bri and Rog getting up there with Adam Lambert and bashing out We Will Rock You, in exactly the same way that I will have no issues with hearing Geddy and Alex doing The Spirit Of Radio with Aneke on drums. The same way that McCartney still gets up there and does Beatles songs at the age of 83. Their legacies are no less secure for them still wanting to go out there and play their songs.
For me, I just want them to get the setlist right - the classics, of course, but also make sure their newer stuff is represented too!