If there is one single thing that can be offered to help a kidney failure cat, it is diet. Veterinary kidney diets are formulated specifically for this, and give your cat the best chance to keep up a decent quality of life. It isn’t necessarily protein level, or anything else that simple. Raw diets are not the answer, nor grain-free. You are also fighting changing taste preferences - recent research shows that when the kidneys fail, preferences change.
My own 3 elderly cats that had kidney problems ate only Hills and Royal Canin veterinary renal diets, and lived over 3 years each with kidney disease. One of them lived 6 years beyond diagnosis, and passed away at 22.
Don’t be afraid of these diets, and don’t worry that your veterinarian is just trying to increase sales. These diets really do make a significant difference. There are no good alternatives.