Should manufacturers provide circuit schematics & parts list?

Consumers often use home audio gear for a long time—often longer than the lifespan of the company who made the gear. A lot of threads on the Forum are devoted to trying to repair audio gear and requests for circuit schematics. A lot of these requests are futile and go unfulfilled because the original audio gear manufacturer is no longer in business. 

Should manufacturers adopt the practice of providing circuit schematics and parts list as part of their products’ owner’s manuals? How about upon request from the original owner?

Obviously, some manufacturers might balk at this suggestion if their designs are patented or protected by some trade secret. But I suspect those who do would be a small number and is moot in any event for a defunct company.