Based on the provided data, the number of major air carriers in the US stood at 18 in 2023, with the same value forecasted until 2028. The data shows a decline from 20 in 2013 to 18 in 2015, maintaining the same number up to 2023. The year-on-year variations highlight a significant 10% drop in 2015, and subsequent years have shown stability without any major changes. The Compound Annual Growth Rate (CAGR) over the last five years indicates no growth, reflecting a steady state in the number of major air carriers.

Future trends to watch for:

Potential impact of economic fluctuations on the airline industry.
Technological advancements and their influence on airlines' operational efficiency and consolidation.
Regulatory changes and their implications for market entry or consolidation.
Environmental regulations that could shift the market dynamics.