Louis Gritsipis insists he will never sell his diner in Hell’s Kitchen, although developers have had their eyes on it for years.

“I’ll never retire,” Mr. Gritsipis, 79, said. “If you retire, you’re dead.”

And why should he? “This is my palace,” he said after a recent lunch service at 42nd Street Pizza, the old-school Greek diner on the ground floor of his four-story, white stucco building. People come for pizza by the slice, or choose from the 220-item breakfast-lunch-and-dinner menu.

But Mr. Gritsipis’s kingdom is under siege — from rising expenses, changing tastes and developers who are trying to buy smaller and more unusual lots to assemble enough land for residential and mixed-use projects. His building, near the ultraluxury Hudson Yards development, is surrounded by glassy high-rises occupied by transient renters and owners, few of whom order gyro platters.

Such are the forces encroaching on the city’s vanishing diners. While their disappearance has been lamented for years, diners along the margins of Manhattan and in parts of other boroughs previously thought impervious to redevelopment are closing because of increasing rents and enticing offers that are hard to pass up. While Mr. Gritsipis’s diner is in a four-story building, many are the lone tenants in single-story buildings. The land and unused development rights above them are lucrative, as a developer can use those rights to gain more height or bulk for new towers, many of them luxury apartments.