"After it burns out, the answer is: so what? Most people are replacing computers much more quickly than 5 years, and even if they didn't, performance would just revert back to "normal", non-cached variety."
Better check those stats again, you will find that most companies and casual users don't upgrade that often. And this "feature" sure isn't going to be a $1 cost to add. If mobos using this cost 30% more, and it lasts for 3 to 5 years (depending on write endurance, write amplification, etc.), then it is a waste for anyone but enthusiasts.
"Most motherboards also have a soldered battery to keep the CMOS alive, and eventually those fail too."
Yeah, but replacing a $1 battery is cheap. And when it dies you don't lose system performance.