What Will Replace PDF?

Actually, PDF is the best there is -- it may not be perfect, but nothing else is as "reliably consistent across platforms" as PDF. Your assumption that it "should be replaced" is not widely shared.

Microsoft tried to promote its very good, open, and well-documented XPS format, but it didn't take hold.

Microsoft's XPS format is one of the few direct competitors. Then there was Yann LeCunn's DjVu format which takes the radically different approach of only being a format for encoding documents as images (as opposed to text and vectors)

It's much easier to call out PDF for what you hate about it than to make something that covers the same use cases better.

ePub seems a posible contender, thouh many individual pubs are horribly formatted.

PDF remains highly usful, particularly in its fixed pagination. Though small devices fare poorly. Tablets are about perfect, form-factor wise.

Another option is publish-to-endpoint, on demand, allowing selection of, say, PDF, HTML, or other preferred format.

Anything you replace it with will, over time, suffer the same problems. If it becomes popular then it will have vulnerabilities. As technology and its uses change, it will need updating to match and so become versioned and have legacy features and complications in implementation. That is just what happens to all formats. Many people just use PDF as a simple way and showing documents and printing them out on standard office printers.

What about HTML+SVG? Where HTML is merely and atypically used to bundle multiple embedded SVGs, and the SVGs are already prerendered as SVG doen't support refloating and paging? Though do we really a new print-oriented, non-responsive format?

One advantage PDF has over many of the alternatives being suggested here is that it’s not read-only. I have yet to see a fillable form in an ePub doc for instance.