Roffa: Having spent the last 24 years involved in light aircraft ownership, had a PPL for 39 years and spent 41 years in Air Traffic Services, the last 36 of these 'rubbing shoulders' with the light aircraft and private/recreational flying community, I accept that you are obviously far more knowledgable than I on the subject of Mode S, but I can't help being sympathetic with the viewpoint of current owners.
From an ATC point of view, it's nice to have (but is it really necessary?), from an aircraft ownership viewpoint it's an expense which in my opinion is not necessary when you only intend to operate a permit aircraft in VFR outside controlled airspace. Rather like radar/radio on a privately owned boat, if you're going out to sea then by all means have it; if you only intend to cruise canals or inland rivers, why bother?