Sure, there will always been some legacy vehicles on the road for people willing to spend the money to keep them operating.
But as fossil fuel cars leave the road, so will the economic interest in keeping gas stations operating. The economic pressure will lead to conversion, just to ensure market share. Some places will take longer than others.
Let me use a car example - you can still find people who own cars without seatbelts. The law only went into place in 1968. But as far as the market itself is concerned, we've moved on. Those people and those cars are outliers, pretty much irrelevant to any conversation about vehicle safety.
If you want a more modern example, use airbags. The law was passed in 1991 and fully in effect by 1998.