There are differences and at the moment the two class experiences are most certainly not like for like.
Apart from the hard product/seat (For BA on A350 currently only) the seats remain different and F is better than current J.
The staff/pax ratio in F vs J is superior.
The toilet and personal space ratio is higher.
Cabin generally smaller, quieter for sleeping
The food offering is better.
For BA at LHR T5 the lounge offering is better (eg CCR) and the new(ish) First Wing private security channel is a great advantage. The J lounges at LHR are often a 'zoo' .
Priority disembarkation (eg F off first) is great at places like MIA and JFK where security is a nightmare
Others airlines differentiate even more eg LH private limousine ground transfers
FF tier point accumulation is higher in F than J
For those that appreciate these things (and often the fare difference is not massive) why would you not?