Because there needs to be a sweetener for the most premium and spend your travellers. Imagine swapping out F on a hi-J 747. The "best" you could be onboard is one of 100 bedsitters onboard.

I see a trend whereby "First" may get amalgamated into J, but some premium / soft / ground services are offered at a premium or to top tier flyers.

Exactly as done on the Eurostar - an unpublished tier, private cabins in Premier only available to top tier customers, and the personal greetings and service of the onboard steward.

We may be a number of years away from that on BA though without total reconfiguration of say the 747 fleet that's staying. But say, Qatar, with the Q Suite product could perhaps have some differentiated service standard. I'd bet a celeb or prince would see an improved onboard service there...