(Perhaps I'm nitpicking, but...)

As ſ does not occur in modern English, many people write f as ſ, and would not find anything awkward with using ſ as f. However, it does bother some who are used to reading ſ.

The f-to-ſ substitution occurs most often in cursive handwriting. Below are examples of both; f on the left, ſ on the right.

The problem with using ſ for f, as you've noted, is that ſ is actually a form of s that went out of general use roughly during the Regency period. You can see it in use in documents such as the Declaration of Independence and the U.S. Constitution.

There were rules for where the ſ form appeared; for example, it never appeared at the end of a word. This means that the word Congress was written -- and printed -- as Congreſs. I read enough "ancient" document (not incunabula, or at least not often) that when I see ſ I read it as an s, and it throws me off to discover that it isn't one.

"ſ" is the "long s" used in the round hand (and maybe some other hands).

A person who writes "properly" does not make this mistake.

I don't think this has anything to do with the cursives. It occurs because some people try to write faster by merely joing the two strokes of the "f" together (first the downward stroke then the line). I've seen people use "ſ" for "f" although their handwriting do not resemble a cursive hand in any way.