I will continue to fly BA when their prices and schedules work for me, and for now (pending the expected devaluation of dynamic pricing) I will continue to redeem avios on BA where it makes sense. What I am not interested in any longer is aiming to achieve status with BA.

So the OP question could be rephrased to: what spend thresholds would entice me to consider achieving BA status again?

I reached Gold in 2017 from spending £3000 (includes taxes and fees), and basically all of that £3000 was spent on travel that I would have done regardless of contribution to airline status. Importantly, the majority of that spend was not on BA.

I can accept that I successfully gamed BAEC and getting Gold from those flights in 2017 was overly generous. I would have been happy with Silver. So allowing for inflation, I would say that if the new threshold for Silver had been announced at £4000 (which excludes taxes) I would have accepted that.

Using the old TP proportions that would mean 2k for bronze, 10k for gold and £233,333 for LTG.

However that is not the only problem, the other issue for me is the gutting of earn rates for many oneworld flights.