The thing is how many other customers do they have that just accept a poor quality service.
That's why the churn rate is so high in the broadband sector.
People cease & reprovide at the drop of a hat based on internet forum comments, chats with friends or whatever..... not realising how intertwined the whole industry is, and how a large part of performance remains dependent on BT and the quality of the tail to the exchange (not to mention contention, traffic shaping and all the other fun and games the providers play).