This is a very interesting debate and fully illustrates the great rationalisation that took place after the Beeching report, which in my opinion has proved to be far more of barrier to increasing train services than the Beedhing closures.
Totally agree that the rationalisation that took place after the Beeching report has proved to be a big barrier to increasing train services, but we should not forget that the report itself took out lines that would be useful today because it was thought at the time that they were "unnecessary duplications."
Also, the report left open the option of closing the Southern route to Exeter at some point in the future on that very basis.
Ironic, considering today's official view of the line, and the proposed infrastructure enhancements.
Dont forget also that several line and station reopenings since Beeching have been considered success stories, and I do think that further re-openings would be likely to continue that trend.
I feel that a mix of rationalisation reversal and line re-openings would be the best way of expressing the argument for line-reinstatements that you rightly say we should make.