The £84k over the first 7 years of employment cannot be underestimated. It's akin to a £12k tax free increase onto your annual salary. In other words, to have the same disposable income in real terms working for another airline (assuming you had the debt of an integrated course to service - which most do), you'd have to be earning considerably more. Such an amount is not realistically ever going to be achieved at such a junior level in any airline. And that's all excluding the extra flight allowances, which are realistically going to be better at BA than anywhere else. From a financial point of view (and very probably otherwise too), the BA FPP cadets are jumping a decade up the ladder over their similarly qualified peers at other airlines. Even then, those other guys are some of the lucky ones - having managed to actually find an airline job at all.
But even irrespective of the above, one of the most crucial elements to the BA FPP is not necessarily that it pays better. If pay was the only driving force for the successful ones then they are going into the wrong profession in my opinion. The most critical element of the BA FPP is that it enables an aspiring pilot the opportunity to train at all, with as close to a guaranteed job at the end of it you're ever likely to get in today's aviation industry. I read about one young lady who made the cut (via OAA if my memory serves me correctly), who would have otherwise started an integrated course the previous year had she been able to secure the loan - which she couldn't. So for an applicant like her, the BA FPP appears like it was the only feasible route into the industry short of a lottery win.
It's not hard to see why most people who know anything about anything on this forum can recognise this to be an opportunity of a lifetime. And BA should be applauded for making it about the quality of the applicant rather than the size of their wallet.