As you know, the heads of these printers are in the realm of nano-technology. Yes, you can put any grade of gasoline through many automobile engines. It's not comparable to an x900 printer. As for the service manuals, they consider those to be proprietary for reasons that should be obvious, available to certain people on a need-to-know basis and under strict NDA. They are within their rights to do that. They are also within their rights to determine user access to components - indeed, how much equipment is there on the market with a warning "there are no user-serviceable parts in this product" or some such. I'm not passing judgment on any of this, I'm simply pointing out that there is nothing unusual or untoward about it.
The extended warranty is a pure unquantifiable insurance risk, because we don't know two fundamental things determining the value of the risks: (a) their various probabilites of occurrence and (b) the costs of the consequences if any of them were to occur. With the benefit of 20/20 hindsight, we know for sure that if we bought the warranty and were fortunate enough to never draw on it we spent a lot of money, and if we didn't buy it and misfortune fell upon our printers we are in for a lot of money and "shoulda" bought it. Otherwise, we are none the wiser. And it is a fact of life that for these printers, the print head is essentially the printer, so if it goes, the recovery costs are high; but the probability of it going??? No-one is saying; however, no matter how low it may be, when it happens to one of us, the probability was 100% and the cost significant. Now, as a rational evaluator of risk, let us pretend for a moment (and this is pretend) they were to give me information showing that the risk of head failure is very low, likely on the basis that only a small fraction of all those sold have ever totally failed. On that basis, given the high cost of the extended warranty I would probably decline it. And that may well have been a rational economic decision based on solid principles of risk evaluation; but then two months out of warranty my head fails. So I'm s.o.l. and it sucks. I don't know what else one can really say, unfortunate as these situations are. All we can do is try to manage the risk as best we can with the means available, and hope the basic quality is in the product to serve us well. And yes, people turn to web forums with their problems, much less so with their successes, so it is not likely the most objective source of reliable guidance on overall, "expected" (in a statistical sense) product performance.