i must be completely missing something ?!?

The alternator belt on my S has never has never come off, in 15 years, 125,000 miles.
I learned in 1971 or 72 How important belt alignment was, when at the tender age of about 18, my 1970 Swinger 340,blew its engine. Thank goodness for 5 and 50 Warranties. I was told, that,that engine had cracked a cylinder head, and lost 4 pistons. The warranty engine was not nearly as strong, but it never blew a belt again.