Full support of commercial anything beyond 10 years is rare. So you have the ability to fully support, today, a product that is 20 years old. Based on failure rate predictions and field failure rate data there is an appropriate stock of fully assembled amplifier boards and/or individual parts in a warehouse for a 20 year old product. That is a sunk cost the Finance department approved for every product and baked into the cost of every product sold?
Or could support consist of a limited supply of custom items backed by purchase as needed resistors, transistors, caps, op-amps etc.? That seems like an approach the Finance department would support.