For me, I think 1st class should be offered on most if not all longer distance services, typically where journey times are over an hour or so. On shorter distance commuter services, or branch lines, I don't personally see the need for 1st class.
What I find strange is that with some operators, the 1st class offer is little different to that offered to standard class. Examples include Southern and Greater Anglia (on the 321's at any rate). If 1st class is to be provided, then (in my view) the TOC's should at least make it worthwhile in terms of comfort & ambiance.
Certainly, for the erstwhile Inter City TOC's, 1st class is a good money-spinner and it would be daft for those companies to sacrifice 1st class accommodation.