You keep saying this, but don't forget to add on (a) time to change, and (b) when connecting onto HS2 time to kill because ticketing will almost certainly be fully train specific and compulsory reservation.
Whereas there are many places from which you can walk to Euston and use a walk-up ticket so no waiting around.
I don't understand this? Are you saying that there will be walk-up tickets available for HS2 from Euston, but not OOC? If not, then it can't possibly make a difference where the service starts.
There are plenty of places within walking distance of Euston, certainly, but I'd be surprised if more than 10% of HS2 journeys had an origin/final destination at one of them.