As others have said, it's a mix of priorities and whether you're paying out of your own pocket.

I think there has been a democratisation of premium air travel over the past 20 or so years. Ironically in correlation with the massive growth of low-cost travel for short-haul. Maybe not on full J fares on BA from LHR but there are many other business class options available, and you'll see many leisure travellers in them.

Maybe peoples' views on the relative value of premium air travel have changed, and splurging on an occasional treat, while far from the norm, is more popular than it used to be.

Thinking about my own personal travel over the years, I've flown J a number of times and F a few times. Only F via redemptions though, and in J, either redemptions, upgrading using Avios (I discovered this before I discovered FT, when booking flights to NYC that purchasing a WTP fare during a sale then upgrading with points was a fab way to do it), or connecting somewhere.

My best realisation for finding better value paid fares was to psychologically liberate myself from BA!