Paxboy
My uneducated guess as to why fuel heaters were discontinued would be:
1) We have better fuel spec these days
2) We can save the weight and cost of these units
Followed by:-
3) It's worked well since we stopped installing fuel heaters ...
And don't forget that the energy to heat the fuel is paid for in increased fuel burn. Only a small proportion of that comes back in the efficiency gain of hotter fuel for combustion