BA is a great opportunity, but its not the only one nor is it the best. Sure the company will cough up the cash and give you a defined career route. But you will be paying back that advanced loan for some time, the command time is around 10 years and for the short term you get get to fly some pretty repetitive routes. Added to this you are just a number, in a company of 60,000 people. Its great for a while, but after some time you lose sense of belonging.
There are other companies that offer sponsorships full and part, that have a better variety of flying in the first few years, faster command promotion times and pay more.
Its all horses for courses. Some people prefer the stability of a large airline with its politics and others a smaller out fit without the hassle. At the end of the day anyone who is going to give you the opportunity to get into a shiny jet has got to be worth consideration.
BA might be the biggest, but it doesnt mean they are the best.