It's an interesting idea. But I'd worry about failure modes. Say it's a Gmail account that gets nuked. Or there's a long-term failure of power and Internet access. But then, I suppose that the warning could include alternatives.
And still, failure would be detectable, because you'd stop getting emails. And arguably that's better than false activation.
But the point is that this is open-source. So you run your own service.
Travel. I don't consistently check my E-mail while traveling. Some paid products allowed changing intervals, but that wouldn't solve that issue.
I don't have a good answer, but will consider some ideas. Perhaps allowing staged responses via different mediums is a good alternative. For me E-mail works but as I've offered this others may have different opinions.
Really appreciate the insight.
Asking for a trusted person to confirm could also be useful