Once an automaker sells 200,000 electric cars in the U.S., the credit phases out. Tesla has already sold more than 126,000 vehicles since 2008, according to estimates by WardsAuto, so not everyone who buys a Model 3 will be eligible.
Exactly why you should never sell, or buy a product based on government incentives. None of the major automakers are troubled by this subsidy cap since very few actually want an EV. Tesla changes this.