There is not 1,500 hours requirement. The requirement isn't about hours, it's about holding an ATP certificate. An airline pilot, holding an airline pilot certificate. Novel idea.

1,500 hours is only one of the experience criteria for the ATP; it has never been a question of whether 1,500 hours is "enough," but of certification, and meeting a minimum certification standard.

For many years, a commercial pilot certificate was considered adequate, and there have been periods when airline pilots have been hired at the bare minimum value for the commercial certificate.

The commercial, however, does not require that an applicant perform to the same standard as the ATP.

A type rating does conform to the same standard as the ATP. Whereas in the past, new airline pilots were not necessarily type rated right away, and later the advent of the idiotic "SIC type" became chic for ICAO compliance, and the withholding of a "full ATP type" held some control by the employer over the employee (giving the employee a thing of value), today pilots train to the full type, to ATP standards, and they're expected to perform at that standard. The only pilot certificate that meets that standard, or requires it, is the ATP.