Setting aside catering considerations for a moment, I wish BA would offer upgrades (space permitting) either at check-in or on-board in return for using up some miles. What's it to BA if someone's backside moves from, for example, a WT+ seat to an otherwise empty J seat?

BA would have 'profited' by relieving you of, say, 12,500 miles, at presumably very little cost to themselves. I'd even 'suffer' the WT+ meal.

The person sitting next to me in the otherwise empty J seat might take a different view of course.

I guess BA would be scared that implementing such a policy would have more people paying for a WT+ ticket in the hope of getting an upgrade on the day, rather than paying for a C/D/I ticket in the first place.