Ignoring the economics, who could possibly believe that FM would fail? I'll bet a lot of the debate of AM vs FM back then, sounded a lot like the debate of analog FM vs HD Radio FM does today.
But there are two big differences between AM/FM and the now FM/FMHD......
1) - FM sounded a LOT better than AM. The difference to the average ear of FM and FMHD is not that noticable.
2) - FM never had any proprietary licensing fees. Now, HD on FM requires a greedy licening fee on BOTH ENDS: a giant one paid by the broadcaster, AND the other paid by the end-user (consumer) to buy the new type of radio. The vicious circle doesn't allow most broadcasters to go HD if there's virtually no audience; and the audience has little to hear if the stations aren't HD. A smart company holding the patent would not be so greedy so fast. But you know how most coporations are these days.... gotta make THIS quarter look good for the stockholders/Wall Street, regardless of how much that will harm the big long-term picture.
3) - unrelated to my comparing point, but today, we also have so many alternate forms of media.