Which is... quite weird, really, because the former opt-outs were mostly fairly minor. I assume they'd be allowed stay out of Schengen in practice; Ireland's not in it and they have an essentially un-closeable border with Ireland (this is, incidentally, the main reason Ireland's not in Schengen in the first place). And realistically the EU has shown no appetite for dragging countries who don't want it into the eurozone; see Poland.