I really don't get the devotion to the GC shown by some.
The London - Manchester/Sheffield traffic could be quite easily fitted onto the WCML/MML respectively (and indeed was and has been), with investment in the infrastructure and rolling stock then better focussed on two rather than three lines (even then the MML has been left as something of a Cindarella).
The GC didn't really serve anything in-between, and at the London end had only joint lines to get it to it's terminus. Joint lines which have been reasonably busy doing more local stuff anyway (Met and what is now Chiltern). The GC could never have substituted for one of the others and so it seems blindingly obvious to me that it should have gone.
The fact that (as someone else pointed out somewhere) we only now now foresee a demand for another route to the Midlands & North only illustrates how wasteful it would have been to have kept it open. Not to mention that new route is going via Birmingham and the West Midlands (Britain's second city and a hugely populous and industrial region), which the GC did not, and will only be after several decades of "rampant" growth and in a national population going on for 50% greater than that of the GCs heyday, compared to an ever declining ridership in Beeching's time (and after for at least a decade).
As for not building over the trackbeds to save them for potential future usage, well that sounds a great idea until I thought about it more. After all, if you decide something isn't needed or wanted, then why not reuse some of it if there is a benefit in that ? Why keep it "just in case" when you've already decided the strength of that "just in case" is so weak you won't even consider keeping it in any form now.
I understand there are degrees of usefulness, and perhaps there should have been a methodology of assessing whether routes might be useful in future and thus worth protecting. Even then, I suspect many still wouldn't have been, and the use of them for road bypasses/housing has probably benefited more people anyway. My childhood abandoned line (the S&D in East Dorset) has since become a road and a busy one at that - benefiting many more (local) people in the process.
With regard to current/potential re-openings (e.g. Waverley just as one example), are they expected to cover their costs or are they only viable with subsidy?
If you removed subsidy from the railways, which in some kind of economic meltdown you might be forced to anyway - what actually runs at a profit?