You may argue that since most bloggers simply volunteer their energies to run their blogs, they could theoretically go on forever. And some will. But most, I suspect, want some sort of payback over time for their efforts — whether it is an income or merely the kind of fame that can be converted (speeches, books, jobs) into income.
Glenn makes the comparison to music - many people play instruments and make music, making no or very little money. From my standpoint, I'm doing this because it's fun and silly and serious and intellectually stimulating. I have no desire to turn this into a book or a career, and it could impact my career in bizarre ways if it became widely known. So, I'm going to putter along in my tiny corner of the web, being amused by the one or two comments a month.