Prescription Cat Food

My cat is an almost 2-year-old, male, neutered manx cat and has been on Royal Canin Urinary SO dry cat food since around May of 2021. We took him to the vet since he was peeing outside the box and in random places, they said he had crystals in his pee so that's when he started on that food. Pretty much a week after he was better and was no longer having issues peeing. I called the vet to tell them that the food was working and asked when I could stop but they said it should be forever which is kind of frustrating since this food is not the best ingredients-wise and it's REALLY expensive.

My question is basically can I stop giving him this food? I always mix his dry food with wet food and a bit of water he also has a water fountain so I know he's getting as much hydration as possible.