Protein is NEW food for soil microbes (vs. the natural recycling that takes place.) Whether the intentially-applied protein is derived from animal or vegetable sources is up to the ethical standards of whoever's applying it. Your point that lawns left alone with just clippings recycled back into them "do just fine". Maybe. But do they thrive? Not usually, unless the soil is an immaculate loam to begin with. Your question: Does a healthy lawn need all that N or all that protein? The key word here is... "healthy". The term "healthy" these days implies "well groomed" in terms of neighborhood standards, at least, right? So I say, YES, the bar is raised. Junk food (N,P,K) needs to be thrown at it on a regular basis to "keep up with the Jones' ", or, the correct decision can be made to begin to feed the soil for those longer-term benefits. Is there protein in grass clippings, leaf mulch & thatch, you say? Sure there is. But think about it. You're essentially turning around the nutrients that were already there in the 1st place, and that level of nutrients may not be overall sufficient to step over the bar of expectations most consumers expect in their lawns. High protein grain meals add food for existing soil microbes, whearas finished compost adds organic matter + beneficial microbes. Neither one is a wrong choice. Or you could develop a marketing program using BOTH. I know you want to do what the customers want you to do. That's understandable! But, if you begin to approach them oveer the winter with a new philosophy, such as: "Feed the soil. Don't just grow grass." I'll bet you'll get positive nods from almost all of your current customer base, and maybe even a higher level of customer-vendor cooperation from them.