That sounds like good logic and therefore the best conclusion that I've seen on this thread - it's a shame that Milton Keynes wasn't build slightly further south, so that the central station was where the WCML and EWR cross, which might have kept the line open (only "might" sure, but you never know)
It certainly seems a more important "loss" than a lot of the others suggested on here - so as things stand, Oxford to Cambridge gets my vote here. The only thing counting against it IMHO is the fact that the whole service from Birmingham/ Leicester through to Peterborough/ Cambridge is only an hourly Turbostar, which doesn't suggest a *huge* market for east-west links in the Midlands - but Oxford and Cambridge are significant draws in their own right, even if there's not a huge amount of actual "Oxford to Cambridge" trade (I agree with
@Taunton regarding the way that people suggest there'd be more actual end--to-end demand than there would really be)
I suppose pretty much everywhere is a diversionary line for another line (other than branch lines).
The countryside is lovely between Hawick and Carlisle but also very empty - and there's not a great deal of demand from the Borders to Carlisle (Carlisle isn't a particularly big place - it just appears big since there's nowhere else of size anywhere near it).
There are diversionary routes from Edinburgh to the WCML (e.g. via Shotts if the main line to Carstairs is closed), so there'd really be pretty few times that you'd be diverting Edinburgh trains via the Waverley.
Or, put it another way, if the GSW had been closed fifty years ago, we'd have regular threads on here suggesting that it'd have provided "resilience" in the event of the main Glasgow - Carlisle route being closed - but how many days in the past five years have Glasgow - Carlisle services been diverted that way? (and the GSW serves Kilmarnock and Dumfries, which mean that it's at least got some regular local trade, rather than just being there in the even that diversions were necessary)