In my experience, BA still provides a better than "decent" level of service for decent fares.
The fundamental problem is that many passengers these days demand ever-lower fares, measured in pennies rather than pounds, and are prepared to accept an abysmal level of customer service to get them. As a result, legacy airlines are forced to lower their fares to match those of the locos (which they do - BA and other major airlines are often cheaper than the low-cost airlines on routes and dates that people actually want to travel) even though their cost base is significantly higher.
People still keep harping on with that idea when it has been disproved again and again.
You credit people with looking at lots of airlines for lower fares but then state well Legacy carriers will have cheap fares on the days that the people really need to fly and they just won't book with us.
The oft used mantra of better customer service didn't show up when thousands of people got abandoned on a couple of Augusts or bags getting left at Heathrow at Christmas or T5 fiasco.
BA does some things well and others lets say not so well BUT it doesn't do everything at a consistent high level that would persuade enough people to still fly with them .