You've just whizzed down HS2 to London. Are you going to take a bus or Boris bike to the West End or the City, or get off at OOC and take a train directly to where you want to be?
Depends on where you are going. If it's the BT tower or UCL or British Museum, then the Liz won't take you to it, and it's easier to travel on the surface than take the tube the short distance to such places. Camden is accessible from OOC, but it's going to be quicker and easier to go to Euston (where you can take the tube or get a bus).
And it's not like there's not 2 tube lines (squishing the SSLs to one line) heading to each of the West End and The City from Euston - and giving more destinations with that one change (Green Park, Leicester Square, Victoria, Charing Cross, Waterloo, Bank, London Bridge, etc would be accessed via Euston) in those parts than OOC can.
What does Stratford improve access to over OOC? East London suburbs, sure. You speed that up, but northern leisure/business travellers aren't that interested in that (and the other way around matters a bit less - people are happier doing changes in familiar places, so the penalty is less). Direct trains to Holborn, Chancery Lane, St Pauls and Bank, as well as Westminster, Waterloo and London Bridge - but they'd be quicker with a change from the Elizabeth line than out to Stratford and double back. The only place, other than Stratford itself (which has direct service from OOC anyway), that would be more useful to have Stratford than Euston that people from the North might go to in reasonable numbers is Greenwich.
*But already connected together by a rail service, unlike Euston/St Pancras.
There's three rail services (again smushing together the SSLs into one) between Euston and St Pancras.
But they, despite being all at least every 3 minutes through the day (unlike the every 10 minutes DLR), typically take longer than walking and add to the cost if not doing the St Pancras-destination leg under PAYG.
But let's include it then - that's an additional tube line to the West End (and out the other way), Thameslink providing a third route to the City (as well as a massive area of the SE), HS1 to Kent and Europe. I'll ignore the mainlines heading north out of London, but they will mean it's quicker via London for some journeys. I think we've surpassed Stratford, no?