Even doing a TP run, you are buying seats from BA at a price BA is happy to sell them to you for - your buffet analogy doesn’t hold up here as you’re not consuming more seats.

Furthermore, with newly-achieved status, you’re more ‘locked-in’ to BA - as are, potentially, your family.

Why would BA not be happy?

Sure, it will cost BA a couple more bowls of slop in GC, but that’s already factored into its pricing model (though your buffet analogy is perhaps more relevant here - someone with a propensity to TP run may be more likely to consume greater quantities in the lounge!).