VAILLANT BOILERS REPLACED FOR NO REASON, so many boilers are replaced for no reason! Scammed? Or Not

100's, possibly 1000's of boilers are replaced every year, but do they really need replacing?
Time and again over the last decade, I've recieved hundreds of calls concerning new boiler installations. Those members of the public, who are maybe a bit synical, or maybe just have a bit of common sense, often call us to get a second opinion on getting a new boiler, after their boilers have been written off, or condemned.
I know for a fact, the majority of gas safe registered plumbers, and heating engineers are pretty concientious operators, and there are some very, very good people out there.
Yet, we repeatedly see plumbers/heating fitters, and especially larger companies, quoting to replace boilers, for little or no reason.
So, it begs the question...
Are they deliberately scamming money from the public?
Or is it a case of plain ignorance? Lack of proper training? Lack of qualifications? Lack of experience? Lack of basic knowledge?

Scammed?