So, what? Debris or, to say better, rocks entering the steel deck and them being thrown at 17,000 fpm and hitting the deck must be producing very much noise. If the field is so rocky, then this noise produced by hitting rocks to the deck will be continuous high amplitude sound waves at more than 100 decibels that is ear disturbing and is this permitted by liability laws? When you design a finish mower you do it considering the field is not much rocky and the stones in the field are small sizes. But when I propose a plastic deck you change the field and you say design of that (plastic) finish mower should also consider fields with much rocks with big sizes. Design parameters should be same when designing a steel and a plastic deck mower. and I don't think steel deck mowers are being designed according to the fields with much rocks of big sizes. If we are going to design such a finish mower which will work in heavy rock field, then the steel deck will fail faster than a, say, "laminated tire" deck. Yes, when saying plastic I am not only mentioning only some known plastic materials. Laminated tires made of used tires too can be used as deck materials efficiently. With such a deck, no noise of hitting rocks to the deck, impact force by the rock hitting the deck will be damped and hence its speed of rock will be slowed, etc etc. See many advantages of such a deck here that a steel deck designs haven't taken into account. Rock rake made of plastic tines on the front edge of mower will only help more on preventing the rocks entering the blade area so that much less number of rocks will be thrown at 17,000 fpm, that is very dangerous to welcome the rocks to the blade rotating at about 3000 rpm. Single incident will put all current finish mowers out of the business.
Ps: lets not forget that current basic deck designs of finish mowers are probably based on older rotary cutter deck designs which were made some decades ago. Since then, no significant progress in the deck of finish mowers.