It certainly would be. Even if that policy ran for say ten years, that would be £10bn invested into new lines in areas where re-connection is much needed.
If such a policy was implemented I would concentrate on the shorter "missing links" first, such as Bourne End to High Wycombe, Colne to Skipton and Uckfield to Lewes. Once most of these "missing links" were finished, then move onto larger projects such as the one discussed in this forum thread.
I certainly don't advocate every single line that was closed should reopen. However there are some very strong cases where reinstatement would be very beneficial. It is those that need to be progressed ASAP, rather than the continual dragging of feet that occurs, especially when the cap is passed round for funding.
However another issue is the TOC's themselves, many aren't interested in running new services unless dictated by Government to do so. GWR for instance aren't interested in running a regular service between Plymouth and Okehampton, SWT had no interest in restoring services to a new Southampton Terminus station. The only operator I have seen, interested in running new services has been Chiltern.
And when you see the constant strife on the railways, the ballooning costs of operating what we have got and of any new build, and the comparatively small number of people that benefit from railways, it is hardly surprising that Government is reluctant to open more. As for TOCs, why would any want to run such uneconomical services you mention on their own account?