A spike in assaults and harassment incidents in New York’s transit system is threatening its ability to restore ridership to prepandemic levels just as it needs to start replenishing its coffers. The Metropolitan Transportation Authority, which carried 5.5 million people on its subways every weekday before the COVID-19 outbreak, faces a harsh reality: Even with $14.5 billion of federal aid, it must plug an estimated $1.5 billion deficit as soon as 2024 if ridership fails to hit prepandemic levels. That scenario would bring back financial strains that could have dire consequences for the future of the nation’s largest transit system. At stake is the MTA’s $51.5 billion capital plan, which would improve access for the nearly 1 million New Yorkers who identify as disabled, expand service to underserved neighborhoods and replace aging signals that cause delays and limit service. The key to it all, however, is revenue. The authority must …