Done to death I know, but the last I was aware Mode S is only required in NOTIFIED areas of Class A airspace, ie not all Controlled Airspace.
This, at the moment, I think is limited to the London TMA, which very few of us will be transitting anyway. What remains to be seen is how many other areas of Class A will become "notified for Mode S" in the future. In short, Class D zones and most airways are still available with Mode C only.
Of course, please correct me if I am out of date.