I'll often purchase first class for leisure trips between Sheffield and London, for three reasons, first, the seats are bigger, I'm 6ft 4 and built like a brick outhouse, plus it's always less busy and also when I travel it's often only 10-20% more than a standard advance fare.
I feel that first is often seen as a means of getting a seat on busy services, in particular up here on TPE, but probably also on the London commuter services - I'm not sure it's first where the problem is, it's the ability to reserve seats in a useful manner and the detrimental impact that cheap advance fares have on walk up fares and season ticket holders, who either pay far in excess of advance or suffer the risk of not getting a seat when paying upfront for a season ticket.