Sometimes you just have to let things play out for awhile: More than 15 million people have signed up for health insurance plans offered on the Affordable Care Act’s federal marketplace, a 33 percent increase compared to the same time last year, according to preliminary data released by the Biden administration on Wednesday. Federal health officials project that more than 19 million people will enroll in 2024 coverage by the end of the current enrollment period next month. That total would include those who gain coverage through state marketplaces, continuing the record-setting pace. Despite a recent warning from former President Donald J. Trump, the front-runner in the race for the 2024 Republican presidential nomination, that he was “seriously looking at alternatives” to the Affordable Care Act, the latest surge in marketplace enrollment is a testament to the law’s enduring power. Legislation passed earlier in the Covid-19 pandemic increased federal subsidies for people buying plans, lowering the costs for many Americans. The Biden administration also lengthened the sign-up period and increased advertising for the program and funding for so-called navigators who help people enroll. […] On Dec. 15 — the deadline to sign up for coverage that begins on Jan. 1 — nearly 750,000 people opted for a marketplace plan on HealthCare.gov. It was the largest single-day total yet. Dr. Benjamin Sommers, a health economist at Harvard who served in the Biden administration, said that improved outreach helped explain the record sign-ups. “I’m pleasantly surprised,” he said. With years of increased subsidies, he added, “it might be…