The issue here is that cat treats are not food. They're treats, made for feeding in addition to balanced food, not in place of it. According to veterinary chain VCA, "Treats should never provide more than 10% of a cat's energy/calorie intake, and a 5% target is better." This is directly as a result of the lack of "complete and balanced" nutrition from cat treats, which are about as nutritionally balanced as a bag of potato chips.
To my knowledge, there is exactly one exception to this: Temptations. As treats go, these are, in fact, the healthiest option if you are replacing food, as they are the only brand I have ever found that is, in fact, nutritionally balanced to serve as food. Anecdotally, I know of aging cats that have survived for years eating a diet of exclusively Temptations, while I personally have used them as a primary food source for cats who are unwell and resisting other food (returning them to regular food in stages as they feel better). Comparing the information on the back of packaging from standard and Temptations treats, you can see this yourself in the feeding instructions:
If I knew of a single other "treat" that doubled as a food, I'd happily recommend it here, but ultimately, after feeding a very wide number of treats over a long number of years, these are the only ones where I've seen this additional labelling. Thus, this question is an exception where I will recommend specifically one brand over a particular class of product--there just isn't any other option on the market at this time. They're not particularly affordable to offer as food, but in a situation where a cat is resisting regular foods, they're a good fallback option.
If you desire an option for supplementing moisture from the wet part of the diet in addition to the dry, you might try one of the puree tube treats that are becoming increasingly common (the "original" brand is Churu, however, most treat brands are now offering their own spin on these treats). These, like any other treat, are not balanced as a food replacement, but can serve as supplemental hydration for your cat while you and your vet work out how best to treat him.
Ultimately, however, in a true critical situation, the best option is what your cat will eat, with consult from your veterinarian. I have subsisted cats on plain cooked chicken and/or Nutrical for short periods, as the primary mission is getting food into them, followed by balancing the food as soon as possible following the initial battle. If your cat is eating Temptations, you at least have an advantage already in that he is eating something nutritionally complete, and you can work back to the original food from there.
