There is probably no cost/benefit of Mode S to GA which is why I would not support it as mandatory for GA VFR.
But as 2D suggests the argument becomes more complex if a PPL is to be allowed into CAS, which he is, and I don't think many PPLs would support a tradeoff of that sort (i.e. no transponder and Class G only). This is before you get onto the poor nav skills (unsuprising given the PPL syllabus) of many PPLs and any CAS infringements just give the powers to be more ammunition.
I don't believe asking every G-reg owner/operator to spend say £2k max (which is what it will be by the time it is a requirement) is by far the biggest threat to GA; it is a relatively miniscule amount on the scale of what it costs to operate a certified aircraft.
And the chances are the cost will fall substantially further because if mandatory the market will be big enough to be worth looking at by a lot of smaller players. At present the GA avionics market is tiny because most of the stuff is purely optional and appears only in new £150k+ planes, the sales of which are miniscule. Also most avionics comes from very big firms who have to recover their ludicrous fixed costs from very small sales, but in reality the market would be much better served by smaller firms. It costs about £200 in parts and direct labour to make a Mode S transponder (I work in electronics) so I am optimistic.
If you believe the future of GA is in PFA and microlights (which I know many do) then look at where the income of the average GA airfield comes from - you are talking of a large scale collapse, with perhaps 75% of GA airfields closing.
The French DGAC has publicly said that GA in France is facing collapse, but unlike the CAA they are doing something about it.