I've had a few teeth root-canal'd over the years, and I did have one root canal fail after 10-12 years. Ultimately, the root of the tooth cracked and made it extremely painful even though there was no nerve left in the tooth. Longer story:- X-rays didn't reveal an obvious crack, so the first treatment was to perform an "apicotomy" where they go in the side of the gum and seal the tip of the root (not nearly as horrible as it sounds, actually). Unfortunately, though there was an initial relief, the pain came back within 6 months. The dentist and oral surgeons agreed that pulling the tooth and doing an implant was about the only choice. And it worked, I'm 2 years down the road and the implant is pain-free and working great.
Given that the failed root canal caused pretty much constant low-level pain and occasional bursts of eye-watering pain, I can see how it would have a negative effect on long-term health. I never HAD to quit working or anything like that, but I had many days where I sure didn't feel like my old self and wasn't nearly as productive. It wasn't abscessed, infected, or causing systemic problems, just constant pain dragging me down, making me eat and drink less. If it had gotten infected, I'm sure it would have been much worse.