Most major composers started as minor composers. Commercial patronage or sponsorship is hard to come by; so where better than a cathedral choir, with (virtually) unpaid but professionally trained singers to help composers cut their musical teeth. If a composer's work doesn't get performed then he/she cannot be called a composer.
He is right that we must persuade the educational system to embrace music more; but to suggest that cathedrals do not provide exciting music is condescending and ignorant. English cathedral choirs, boys or girls, represent the apogee of choral excellence. To let them moulder in their closes would precipitate the literal dumbing down that Mr Kennedy seems concerned about.