Brian pointed me to an article at the Chronicle of Higher Education about Lawrence Golan’s Supreme Court fight against copyright expansion:

The dispute that led to Golan v. Holder dates to 1994, when Congress passed a law that moved vast amounts of material from the public domain back behind the firewall of copyright protection. For conductors like Mr. Golan, that step limited access to canonical 20th-century Russian pieces that had been freely played for years.

Re-copyrighting works isn’t a technical or minor matter.  It has a huge effect of performing musicians like Golan:

When a work is in the public domain—that Puccini opera, say—an orchestra can buy the sheet music. Symphonies typically cost about $150. And the orchestra can keep those pages forever, preserving the instructions that librarians laboriously pencil into scores. But works under copyright are typically available only for rent. And the cost is significantly higher: about $600 for one performance. With the flip of a switch, the new law restored copyright to thousands of pieces.

For big-city orchestras like the New York Philharmonic, that change is like a "mosquito bite," Mr. Golan says. But Mr. Golan’s [University of Denver] ensemble gets only about $4,000 to rent and buy music each year. That means it can perform some copyrighted works but must rely on the public domain for about 80 percent of its repertoire. And $4,000 is relatively generous. Other colleges might have only $500 to spend on music. When the Conductors Guild surveyed its 1,600 members, 70 percent of respondents said they were now priced out of performing pieces previously in the public domain.

As Brian asked in his email pointing me to the article,

it sounds like US courts have always been willing to extend copyright protections…will there ever really be a "public domain" in the future for works published/created after World War I?

Unfortunately, the jury’s still very much out.