Is there a simple way to describe the phenomenon when usage of a word or expression changes in such a way that it leaves the original concept without a word to describe it?
As usage evolves, sometimes the a new meaning orphans an existing definition. For example, cadence is used when schedule, period, or rhythm is meant.
"We are on a weekly cadence for planning meetings" vs. "We are on a weekly schedule for planning meetings."
At some point, we will no longer have a word meaning "a sequence of notes or chords comprising the close of a musical phrase." There is no alternative word for this definition of cadence.
Another example: "Begging the question" is used all the time to mean "suggesting the question," rather than its original definition of the logical fallacy of assuming the conclusion in the premise.