The result is that Cadillac, a brand that had begun to transform its stodgy, retiree-only image during G.M.’s post-bankruptcy comeback, is struggling. Cadillac’s annual domestic sales had rebounded above 180,000 in 2013, up from about 110,000 in the recession doldrums of 2009. But sales plunged to 156,000 by 2017, even as the German brands were soaring. In the closely watched luxury race, Mercedes rode its own S.U.V.-heavy lineup to 337,236 sales in 2017, while BMW found 305,685 buyers, nipping Lexus’s 305,132.

Now, GM’s leadership has replaced the South African-born Mr. de Nysschen with Steve Carlisle, the president of G.M. Canada, who joined the company 36 years ago. In a statement, Dan Ammann, G.M.’s president said that Mr. de Nysschen had set “a stronger foundation” for Cadillac.

“Looking forward, the world is changing rapidly, and, beginning with the launch of the new XT4, it is paramount that we capitalize immediately on the opportunities that arise from this rate of change,” Mr. Ammann said.

That 2019 XT4 finally goes on sale this fall, priced from $35,790, and it will need to elbow its way through a crowded party of compact S.U.V.s. A three-row XT6 is also in the works, but George Peterson, president of the research firm AutoPacific, did not expect it to arrive before 2020. Mr. Peterson already counts some 38 small crossovers in the marketplace, ranging from $20,000 to more than $90,000, and said that count should soon reach 45.

“Cadillac is recognizing that it’s a crossover world, but they’ve been slow to get off the dime,” Mr. Peterson said. “They’ll be launching into the teeth of a hugely competitive battleground.”

Cadillac may have had a chance to move more quickly.

Following his hiring, Mr. de Nysschen tried to persuade G.M.’s leadership to move S.U.V.s to the front of the development line, according to two company officials who spoke on the condition of anonymity because they were not authorized to discuss internal matters. Traditional sedans and coupes that were already in the product pipeline, including the flagship CT6, proceeded on course. Despite wildly positive reviews, they have mostly fallen flat with S.U.V.-besotted consumers.