Also worth noting that the Modernisation Plan didn't envisage electrification from Woking (well, Pirbright Junction) to Southampton and Bournemouth, so that was a Beeching add-on.
The Glasgow Suburban electrification had originally been based vaguely on the extensive Inglis Report proposals from as far back as 1951 but only the Airdrie-Helensburgh and South Side group of lines had actually been authorised. These were largely complete by the time that Beeching took over, having been started in 1958, so were very much 'winding down' rather than 'rolling'. However, Beeching did re-start the programme with the Glasgow-Gourock/Wemyss Bay scheme.
Sensible phasing of the King's Cross suburban routes indicated getting the Victoria Line and Finsbury Park-Moorgate routes sorted out first. Marples had given the go-ahead to the Victoria Line after pioneering use of cost-benefit analysis (a technique which Beeching also embraced). The scheme might have been authorised in the late 1960s after Beeching had gone but had to wait for the Heath government.
Beeching also presided over the tactical 1,500V dc extension from Sheffield Victoria/Rotherwood to the new Tinsley Marshalling Yard.
All very sound decisions in their day.
At a more general level it was patently obvious that with the haemorrhaging of traditional freight and passenger traffic in the face of road competition that BR had already (recently) ordered broadly enough diesel locomotives and DMUs to meet almost all of its 1960s traffic requirements on lines that were going to survive. There was little financial appeal in dumping this new kit that was then less than ten years old. The good old British Transport Commission had already made that mistake once with a large fleet of post-war new steam locomotives.
I do think in this country, if something doesn't go to plan, rather than learn lessons we give up. This was as true of the post GW electrification landscape as it was of the modernisation plan schemes.