The erosion of Hong Kong's "specialness" and its rapidly aging population could see its trend growth more than halve over the next decade and even plunge to zero in a "plausible" downside scenario, rating agency S&P Global estimated on Monday.

Hong Kong's long-term economic prospects faced risks from the deterioration in the U.S.-China relationship, resulting in a decoupling of the U.S. dollar and Chinese financial systems, the agency said.

A second risk was an accelerated financial opening of mainland China that would lessen Hong Kong's role as a global financial center.

"Hong Kong's specialness is being eroded," S&P said in a new report. "A plausible downside scenario could see trend growth of zero in a decade," it added, saying its "baseline" estimate was for a fall to 1.1% by 2030 compared to 2.7% back in 2018.

A downside scenario would be an acute or chronic deterioration in macroeconomic stability and prospects.

"We see the most likely catalysts for each of these scenarios being financial decoupling from the U.S. and a gradual, persistent rise in domestic policy uncertainty, respectively," S&P said.

Washington is reviewing Hong Kong's special status under its legal system in reaction to China's decision to enact a new national security law for the former British colony, despite its wide-ranging autonomy and guarantees of freedoms not enjoyed on the mainland.

"While the economic impact of such efforts could be very large, at this point we see this as an extreme tail risk rather than a plausible downside for the Hong Kong economy," S&P said.

Hong Kong's financial industry has long been a cornerstone of the city's prosperity. It accounts for nearly one-fifth of the economy, almost double the level just before the 1997 handover from British rule to China.