The boards are a minuscule part of the cost, both monetary and environmental, of the items in question. The cases and PSUs, less so. You can buy the assembled PCBs for a Chinese Quad 405 for £30-40 or so. Shipped to you. That won't pay an hour of a tech's time. It would make sense, both fiscal and environmental, to make replacement boards available for drop-in replacements in the same way that car parts come as bolt-on assemblies. The manufacturing savings will pay for the provision of a spare board or two. Christ, you could give them away for free. An amplifier's capacitors last 20-25 years. Buy a spare board with your new amp, and you are done for 40-50 years with one half hour repair. That's good enough. A vanishingly small proportion of consumers keep anything even 10 years, so 50? We're done.Not the case at all. SMD are much more difficult to repair. The damage to PCBs is mostly down to the quality of the circuit boards and their design. Poorly designed and made circuit boards are very easily damaged. Good well designed and well made circuit boards are significantly more robust and much less likely to be damaged during a repair. Most SMD boards are just thrown away and not repaired as it costs too much to repair them than it does to replace the board.
The thing to note these days is the availability of component parts. As more products go towards SMD, conventional through hole components become harder to find. The whole way our lives have been manipulated and forced to use computers and computer tech means that items that used to have a 10, 20, 30 year life span, now only lasts a few years, so a new replacement item is required. How can this be environmentally friendly? How long can we do this before we strip our planet of its raw materials?