I read something like that recently, it said cats fed on low protein diets could have muscle wastage
Therefore, I am not too concerned that Clapton won't eat any renal food.
Any senior food has around 30% less phosophorus than regular adult food so this is a cheaper way of reducing, although nowhere near as low as renal food.
To give you an idea - these are very rough figures for Phosphorus content in wet food.
Adult 1.3%
Senior 1%
Renal 0.3%
Clapton is doing well on a mix of Fortekor and senior food - although he also has adult food as a treat (felix in gravy)
He also has high quality protein - ie roast chicken - a little bit every day.