As the only person around here that is top tier on both airlines I've gotta say that what BA has done is much worse than what DL has done, part of it has to do with the nuances of the programme, but it was already in many ways infrior to DL, now they are creating a perfect storm. The equivalent would be if DL were to.....
Not only introduce MQM's but to DOUBLE the amount of premium treavel you would have to fly to requalify (so imagine needing 200,000 MQM's to get there, that's MQM's not miles).
Get rid of PMU's without making a formal announcement ( they never hasd an exact equivalent, but have elinimated three differnet Gold bonuses recently)
Reduce the amount of miles that you earn per flight by appx 20-35%.
and
Increase mile redemption levels by as much as 300% for some routes (and none increased less than 20% many much more).