Trains should be designed so that there is not regularly unused 1st Class provision while standard is full, obviously it will happen sometimes due to disruption or one off events. If this is the case then no it's shouldn't be declassified.
That said other than the ECML, MML, WCML, GWML and Eurostar I'm not convinced by the need for 1st Class. Certainly all former NSE services should be standard class only to maximise capacity. There is rightly a lot of work being done with Thameslink, Crossrail etc to increase capacity, but the increase in both seats and more significantly standing capacity by removing 1st class sections would be far cheaper.
That said other than the ECML, MML, WCML, GWML and Eurostar I'm not convinced by the need for 1st Class. Certainly all former NSE services should be standard class only to maximise capacity. There is rightly a lot of work being done with Thameslink, Crossrail etc to increase capacity, but the increase in both seats and more significantly standing capacity by removing 1st class sections would be far cheaper.
