In order for True Love’s Kiss to work WickedRegal, it has to be reciprocated by both parties. They have to truly love each other. It’s why when Belle lost her memory in season two, true love’s kiss didn’t work because she didn’t remember that she loved Rumple. It has to go both ways.
The minute Snow and Charming stop loving each other, that’s when true love’s kiss will stop working for them as well. Because they wouldn’t truly love each other each other anymore.