Contrary to your subject title, it is not only the page ranges issue that has "come apart". At this point in the subgroup's discussion, ALL indexing proposals -- see/see also, sort order, page ranges -- are back on the table. The old points of conflict are being revisited, and we are back to square one, questioning every proposal.
Indexing is a contentious issue. One of the original architects of DocBook once told me she was glad she did not have my task: it was just too ugly. The existing indexing proposals are a mix of hard-fought compromises that nobody involved was fully satisfied with. Now that the issue is reopened, all hell has broken loose again. That's natural for an issue with so many unsatisfied opinions.
It's probably not so much one issue that has "come unglued", but perhaps this TC's process. The votes to approve these proposals were unanimous. By contrast, we never voted to reopen any of indexing issues -- let alone ALL of them -- and never agreed on any kind of charter or limits. Yet right now, anything goes. That is what bothers me: nothing is binding, anything can be reopened at any time by anyone without a vote.
Some people here will be happy drop page ranges from the standard. That's easy enough to suggest. But I'd point out that we have not voted on that idea either.
Chris