The phrase "diminishing share" is correct and can be used in written English.
It is often used to refer to a declining or decreasing portion or amount of something. Example: "The company's market dominance is being threatened by their diminishing share of the industry's profits."
They took their diminishing share of the family riches with them.
The newly emerging climate debate prevented the coal industry from acting as a substitute for the diminishing share of nuclear power.
But state appropriations are funding a diminishing share of the cost of that education, so tuition and student debt are rising.
Among consumer-goods industries, textiles are highly developed, though they provided a diminishing share of total exports in the early 21st century.
There would be no "taking back control" for most working-class leave voters, just less control over their diminishing share than ever.
Despite a diminishing share of the car market, Renault narrowed its 1985 losses to $1.56 billion, from $1.79 billion a year earlier, Renault said Friday.
The growth of mobile has led to the diminishing market share of Windows, which is now at 35percentt, compared to the 45percentt share of Android and iOS.
Questions remain whether Palm's shift toward software and services will counterbalance the company's diminishing market share.
The new product, still not robust to customers' conditions, is eventually forced into the marketplace by the pressures of time, money, and diminishing market share.
The result is denominationalism in which each Protestant church tries to be just different enough from other Protestant churches to attract an increasingly diminishing market share.
The magazine added that she is "in the hot seat" because of "languishing stock at the snack-and-soda giant and diminishing market share".
