I know heritage designations focus on the facade -- I live in a heritage district and I put a lot of work into keeping our house maintained both inside and out. There is something worth preserving about our house because previous owners have also kept it up. But if you walk inside 584 Church, you won't find anything worth preserving. You might as well do a facadectomy. It was probably a fine building once, but I don't believe it's worth saving just because it's an old building.
To be honest--it wouldn't matter, anyway, whether there was anything worth preserving inside, because the likelihood of its interior covered under a listing would be slim-to-nil. As would be the case, for that matter, for most of what exists within your heritage district, whatever your loving/caring decision viz. your own house. Yes, even preexisting total gut jobs within your neighbourhood wouldn't be invalidated as "heritage"--if there's nothing worth preserving inside, it doesn't make it a worthless knockdown, especially if what remains still contributes positively to the streetscape.
So, in this case, even if it's little more than a facadectomy in practice, there's still a good argument for listing/designation and retention,
as part of an ensemble rather than as a prima-donna landmark. And as you can tell from this discussion, it's about an urban ensemble rather than about prima donnas under threat.
I mean, I know what you're getting at, because the heritage community is very mixed about the facadism gambit. But let's not get hyper about it; by your logic, the retained Yonge storefronts of BCE/Brookfield Place are expendable because they're just shells, rather than "complete entities" a la the Bank of Montreal/HHOF...