I don't get why people keep repeating that Chiltern run an insulated line. Their lines are shared with FGW, CrossCountry, London Midland, several freight operations and of course London Underground, who own and operate a large chunk of the Aylesbury line.
The one train a day to Paddington has to deal with FGW, true, but most of the Chiltern lines are relatively conflict free, especially the critical part going into London. Contrast this with East Coast who have to share tracks (on the main stretch from London to Leeds/ Edinburgh) with:
First Capital Connect: London to Peterborough
First Hull Trains: London to Doncaster
Grand Central: London to Northallerton
East Midlands Trains: Peterborough to Grantham (IIRC the southbound EMT service has to cross the northbound ECML track twice?)
Northern: Doncaster to Leeds, Newcastle to Morpeth etc
Cross Country: Moorthorpe to Leeds and Doncaster to Edinburgh
ScotRail: Drem to Edinburgh
TPE: York to Newcastle
(A lot of these other services are stopping at all stations (such as the ScotRail service from Drem to Edinburgh), meaning an East Coast service only has to be a minute or two late to end up stuck behind a stopper for a long period)
Lots of freight and a few charter paths too, plus a number of other TOCs trains crossing the ECML at main stations (where they only share ECML track for a short period, but eat up paths, e.g. the Northern/ TPE services from Cleethorpes/ Scunthorpe to Sheffield/ Manchester)
Plus there's bottlenecks at places like Welwyn, Newark etc, and conflicts with local services on "non core" East Coast services, plus some single track sections further north too...
Chiltern have
almost a blank sheet of paper to plan their services around - no contest
Always easy to be the underdog and chip into the London - Birmingham market (like Chiltern have done).