Long term support is subject to many, many, variables - several of which have been covered here already.
I think one important often overlooked factor can be the ownership of the company. When a brand changes hands, that is an opportune time for the new owner that is not emotionally attached to what was done 20+ years ago to reallocate those questionable resources. Like it or not, you can understand how and why that happens.
Another factor is uniqueness of the parts. If a diaphragm or seat from 20 years ago is still being used today, or perhaps was also used on an item that has only been discontinued for a few years, then it is easy enough to build a parts kit. (Or at least for as long as the parts hold out.) Also simple enough if the part is a widely used "stock item" that is outsourced from a general supplier rather than a truly unique proprietary part that is/was produced in limited runs for limited uses. I just checked the parts in the service kit for the Aqualung Coustea/Spectrum/SEA 1st stage, and the filter is probably the part most at risk. If it is not a part in current production for some other brand's item, I could easily see that kit disappearing if/when it comes time to do the math on running another batch. Or course, they might very well be sitting on hundreds of that filter right now while only selling a few each year.
So bottom line, there is no such thing as a "one size fits all" rule or reasonable expectation of ongoing support. When buying used, you need to do your homework and remember the phrase "buyer beware." I see items offered for sale on this forum all the time that say things like "new, just bought last year" but haven't been manufactured for 10-15 years. The one generality I will suggest is that standard, overhaul service parts/kits will almost always be available for far longer than replacement parts that are only used "as needed" when something breaks, is abused, or wears out. Unless common to a more current item, those are often limited to "stock on hand" at the time the item was discontinued from the catalog.
I think one important often overlooked factor can be the ownership of the company. When a brand changes hands, that is an opportune time for the new owner that is not emotionally attached to what was done 20+ years ago to reallocate those questionable resources. Like it or not, you can understand how and why that happens.
Another factor is uniqueness of the parts. If a diaphragm or seat from 20 years ago is still being used today, or perhaps was also used on an item that has only been discontinued for a few years, then it is easy enough to build a parts kit. (Or at least for as long as the parts hold out.) Also simple enough if the part is a widely used "stock item" that is outsourced from a general supplier rather than a truly unique proprietary part that is/was produced in limited runs for limited uses. I just checked the parts in the service kit for the Aqualung Coustea/Spectrum/SEA 1st stage, and the filter is probably the part most at risk. If it is not a part in current production for some other brand's item, I could easily see that kit disappearing if/when it comes time to do the math on running another batch. Or course, they might very well be sitting on hundreds of that filter right now while only selling a few each year.
So bottom line, there is no such thing as a "one size fits all" rule or reasonable expectation of ongoing support. When buying used, you need to do your homework and remember the phrase "buyer beware." I see items offered for sale on this forum all the time that say things like "new, just bought last year" but haven't been manufactured for 10-15 years. The one generality I will suggest is that standard, overhaul service parts/kits will almost always be available for far longer than replacement parts that are only used "as needed" when something breaks, is abused, or wears out. Unless common to a more current item, those are often limited to "stock on hand" at the time the item was discontinued from the catalog.
