Just to add that the division between F C and Y demand varies from route to route (and also, with day of week and proximity to holidays etc). Seating configurations of a "one size fits all" type can lead to considerable inefficiencies on certain routes. And if you start op-upgrading your Golds in Y to balance the seating demand, the Golds soon catch on and never pay for a premium seat (leading to the extreme problem that US domestic carriers have got into).
For example, from London to New York, prime Transatlantic destination in the USA, there is extensive demand for both F and C. To Toronto, the prime destination in Canada, there is very little demand for any premium class service, and never has been (even back in the 1960s BOAC had a small subfleet of all-economy 707s used to Toronto). So if the fleet is large enough then you can justify more than one configuration on a type.
The Gulf carriers, surprisingly, were early adopters of this multi-configuration approach, as they bought widebodies for Europe (normal spread of demand) and to India etc (hardly any premium demand).