I think there are two main reasons that the short life span is a big negative: sadness at the loss of a pet, and the inconvenience of needing to go through the find, buy, "what species did I get?" cycle. This won't work for everybody, but I use logic to get around the sadness. When my octopus dies of old age, I remind myself that the wild is a very dangerous place for an octopus, and that I've very likely given my octopus a longer life than it would have had under any other circumstances. When an octopus dies of old age in my tank, it's the best of all possible outcomes, and I'm emotionally at peace with that.
As for the inconvenience, I "cheat" because I catch my own bimacs, for free, and for me it's fun to hunt for them at low tide, so I don't find replacing a bimac to be a chore. I also do what I can to maximize the life span of my octopus. While it is not scientifically confirmed for octopus, there is evidence that many animals have longer life spans if med a minimal diet, and for cold blooded animals if they are kept at the lower end of their temperature range. I think Roy has written that in his lab he routinely keeps bimacs for two years with cool temperatures and frugal feeding. When I caught my current bimac, about 14 months ago, it was more than half of it's adult size already. I keep it at 56 degrees and feed it every two of three days, and only enough to keep if from getting smaller/thinner. I strongly suspect that one can maximize the life span of any octopus by keeping it at the low end of its natural temperature range, and feeding a minimum amount, but enough to allow for slow growth, or for a fully grown octopus, just enough to maintain bulk.