As with most things of this ilk, it's all about how Apple perceives 'risk' vs 'reward'. You are correct in that only Apple knows the true scope of the problem, and, given the overall cost to reputation, has determined that the cost of a repair program will (may) buy them some grace with their customer base.
With that said, I am suspicious that 'correcting' the problem without a true 'correction' to the design is a bandaid on a bullet wound. It might stop the bleeding for a minute, not I'm not convinced that it will do much to resolve the ultimate issue. I am also highly convinced that the 'thin is in' mantra is a large part of the overall issue - in order to make the laptops as thin as they can/do, a new keyboard switch was required. It is also my opinion that Apple has fornicated with the canine on this one.
For the record, I do not have the problem, as I am still rocking a 2013 MBA and a 2015 iMac... however one of my sons-in-law does have the issue, and has pretty much been frustrated by Apple at every turn. This 'repair program' has given him hope, but it's tempered with "what happens the next time this design fails?"...