How does this affect use cases? The explainer mentions polyfilling dialog, but maybe the argument would be stronger without mentioning polyfilling dialog because of the difference in the attribute applying to subtrees and dialog applying inertness from document root except the dialog subtree.
The idea that it makes dialog easier to implement also might be stronger with some clarification—it requires an "inert" property for nodes which dialog also requires, but it doesn't simplify how that property is computed or interactions between things defining that property.
Yes, there's an encapsulation problem with the idea of it switching on and off.
It would be good to have a name for this idea. The HTML spec has "inert" and "expressly inert", and this proposal is a third thing—"sticky inert" or something—because in your proposal it takes precedence over dialogs which aren't inert, despite having inert ancestors, because they're active and not expressly inert.
Untangling these concepts would be good.
I wonder if I'm just confused, that your proposal is a separate parallel concept which has the same effect on focus, etc. as inert does (or disabled for that matter) but exists separately, and I was just confused by overloading an existing coincidental spec term.
I think English works against us here, by the way, because "ert" isn't a thing; there's just in-ert and not in-ert looks like a double negative.