I'm really curious, what would be a (potentially) good reason for offering a 20 year warranty? Bragging rights? Pride?
Is it a positive enough selling feature to offset the long term repair costs?
Good question! One word answer: marketing.
A longer warranty gives the impression of higher overall quality and higher confidence in the product. Car companies (usually ones in trouble, or known in their past for unreliability or shoddy manufacturing) have used this strategy to help sell their new (and hopefully improved) models. Car companies who engaged in this practice are now retracting these warranties back to industry standards, probably for overall cost containment, or because in the (current) strong car sales market, long warranties are perceived as being unnecessary.
In audio, a 20-year warranty helps differentiate from the competition and create the perception of a higher-quality product. In the past, this wasn't really such a bad bet. I have quite a bit of Sumo and Theta gear that's 20+ years old and still working fine, though they are probably do for a re-cap soon (more on this below).
Back then, we were using leaded solder, so the inherent lifespan of the solder isn't a factor--20 years should be well within the capability of leaded solder. That means, in the past, the primary lifespan-determining factor for audio gear was probably the capacitor lifespan. Electrolytic capacitors do dry out over time. This is why some older equipment hums--the electrolytic power supply capacitors have become less effective at, well, being a capacitor. Replace them, and the hum goes away. (Please note this is not a universal panacea--there can be other problems with old gear.)
So, back in the 1980s and 1990s, sure, a 20 year warranty wasn't outside the realm of consideration. Add some cost at retail to cover the additional warranty work, and call it good. Yes, it will increase cost at retail, at least if you really plan on being around in 20 years, but it's not completely crazy.
Today, a 20-year warranty gives me the cold shivers--largely due to lead-free solder, as noted in the chapter. Will the current formulations of lead-free solder last 20 years? If they don't last 20 years, how bad will the tin whisker problem be? Just a short here and there, or thousands of shorts all over the board? Will it fail in 7 years? 10 years? 15 years? Can you add enough retail cost to deal with a possible epidemic of board swaps coming in 10-20 years? Will you have the boards to swap? Will they still be good? (After all, you're probably not going to make the same product for 20 years.) Can you add enough retail cost to simply swap products for the new model? Those are a ton of questions I don't want to worry about answering. So again, excuse me for wanting to be around in 20 years. If this was a flash in the pan, sure, give as long a warranty as you want (well, unless you want to sell the company someday.) For us, we have to be realistic.
For us, I want to be kind of like the service I have gotten from Velodyne on my now 20-year-old 18" servo sub. That's a big, expensive product, with (I believe) originally a 2-year warranty. It has broken twice out of warranty, both times being something wrong with the amp module. In both cases, Velodyne provided prompt service at low cost to get the sub back on the road again (including the latest break, 2 years ago, when I asked them if it was simply time to buy another, given the age of the product.) I completely understand the original length of the warranty (it is a big, moving thing that I'm sure gets a lot of abuse in home theater environments), and I appreciate the low-cost service for a LOOOONG time after warranty. That, I think, is the right model.
Again, others may have different opinions. And I can always be wrong.