Mens' standard business attire has been more or less unchanged for more than 100 years. Historically speaking, is that a long time for one fashion item to endure, and why/why not?

It really depends on how you define "more or less unchanged". (I just had a part in the recent Twitter furor over Beau Brummell and modern menswear, so this has been on my mind.) One hundred years ago, a man's suit coat would have a defined and raised waist, a waistcoat/vest, high-waisted and narrow trousers usually held up with suspenders, and a shirt with a detachable collar of starched cotton or linen (or celluloid, if you couldn't afford the laundry); spats were also pretty fashionable, and hats were required. There are similarities, of course, but it's not as though historians can't date the parts of a suit based on material, color, and cut - as though they're fossilized forms. Pant legs and waists, lapel widths and shapes, side vents and back vents, double-breasted and single-breasted, colors and materials ... there are many aspects of a suit that change according to fashion.

The suit is like the ship of Theseus in the famous paradox - if you replace the boards of the ship one by one as the old ones rot or break, when does the ship become a new ship? Or does it ever? Likewise, which incremental change in men's fashion marks the beginning of modern "standard business attire" and separates it from old-fashioned dress? The twentieth-century loss of the waistcoat, hat, and suspenders is a pretty big thing; the modern sack coat took over from the morning coat/cutaway coat, which replaced the frock coat (to be very brief and gloss over the subtleties). There has been continuous change, some changes more obvious and some less immediately so. (I list a few more in this answer.) The underlying premise is not correct.

But taking it at face value so I can answer - no, historically speaking, it's not. In modern history, we expect "fashion" to consist of regular changes of color, cut, fit, etc. that allow people to be seen as more or less wealthy/high-status/with-it on the basis of fairly arbitrary considerations, but in Europe prior to the Late Middle Ages (with exceptions) this was simply not the case. Clothing was basically made on the same basic lines, and showed status on the basis of expensive materials and ornamentation. Even after the outer clothing began to alter more frequently and less subtly, men's shirts and women's shifts underwent barely any change for several centuries.