If all mechanical/electrical/electronic thingies were defect-free and maintained in timely fashion would they ever break down? How?

I am a victim of this. I had a motorcycle parts and repair shop. As motorcycles became more 'electronic', they became more difficult to diagnose. The industry was not providing the information and tools to do an honest diagnosis. Too much guesswork is not acceptable. I was a mechanic, not a computer repairman.

No. Most people have no idea about  or don't want to perform proper maintenance.   A large chunk of any repair shop is performing maintenance.  Electronic parts are very rarely serviceable these days and need replacement when anything goes wrong with them. 

Any good repairman knows two things.  Maintenance free is a lie and really means " maintenance impossible, parts possibly replaceable" and that nothing is ever defect free. This post was edited by Benedict Arnold at November 26, 2016 3:41 PM MST

Even with proper design, manufacture, care and maintenance, everything wears out in time. Bearing surfaces in mechanical devices slowly wear away; plastic and rubber materials eventually break down. The problem now is that many manufacturers don't want you to repair anything if they can sell a replacement, but on the other hand many owners will throw away items that are in fact repairable - or far more wasteful, are in full working order but scrapped for spurious reasons such as "not the latest" version / style / level of pseudo-sophistication.