External e2e failure ... then what

Reading the knowledge base regarding e2e it seems for even a modest size network that an external e2e is required.
Is this only true of a mesh topology or an absolute once past 21 nodes ?
What happens when the e2e becomes unavailable ?
I’m guessing that short term things run as they were and any link that goes down is down until e2e restored ?
I guess the big question is just how bulletproof, how much backup is needed ?