*if* fitting Mode S to a glider was a £500 shot (it isn't) *and* there weren't problems in most gliders with the batteries being able to power it for long enough I don't think it would the issue it is. For a lot of low-value gliders fitting Mode S will cost as much as the glider is worth, or more. Even for a 15K glider, it's still going to cost 30% or so of the value.
Interested to see the replies from the ATC folks.