Cannot answer many of the questions but many older HMI's were retired because of the ROHS compliance, things like change from lead based solder & so forth. So. there could be a lot of redundant stock out there that contained some of the more hazardous materials, many companies when they stop using a particular part then there is a glut of them that come to the market for example the thermionic valve, most people stopped using them in the 60's or earlier, some types are still manufactured as still in use, some because people want to keep old legacy stuff going & the obvious is the amount of stock that manufacturers do not want that is still availlable.
Most manufacturing companies like the food industry do not have people with the skills or time for repairs, they either send it to a specialist for repair or more often than that they just scrap it & buy a new one, as the systems nowadays are upgraded to keep pace with the latest innovations companies want the latest, it's a throwaway society & just like people they want the latest Iphone. In actual fact, a few years ago I sent off a PV550 for repair, the cost was £10.00 more than a new one from our supplier with our discount.
So as for the question about youtube, as HMI's tend to be mainly for industry then mobody can be bothered to create one as it probably would not be viewed many times.