Erm, Peterborough to Ely/Cambridge is only an infrequent 2 or 3 car train, all of which are normally pretty full with their own passengers and are overwhelmed if used in the way you suggest by LNER etc.passengers.
Disruption south of Stevenage often results in no services to Cambridge/Peterborough either, and I'm not sure that going Stevenage to London via Cambridge would be an acceptable route even in severe disruption.
I didn't suggest anything - I just said 'have the possibility of'.
The reality is that in normal circumstances trains on the ECML (and other busy routes) are moving and distributed over the length of it, with a higher concentration at the southern end.
If there is a major problem that stops them reaching e.g. London, I doubt there's a single place big enough that they could all stop in a platform and wait, so I suspect the best you can do is either leave the passengers on stationary trains (preferably in a platform somewhere), or terminate the trains at somewhere like Peterborough, discharge the passengers then run them ECS to carriage sidings (until they fill up too) or send them back north (if there's crew to do that) to get them out of the way. At which point what do you do with potentially thousands of passengers milling around the station and car park area etc., other than try to get at least some of them to their destinations by whatever alternatives are available?
I would sincerely hope that Stevenage to London via Cambridge would be an acceptable route in the event of severe disruption, otherwise IMHO the 'railway' has an utterly bonkers attitude to customer service (and in my experience railway staff do generally *try* to keep passengers moving if at all possible, even via roundabout routes).
