Another thing to remember about possible "new" battery
technology is whether the glider manufacturers will have any
financial interest in updating their current systems. New &
better batteries may well appear but you can bet the glider
manufacturers will be fitting them in their latest creations. So
for anyone wanting to fly electric in the near future, or now,
your are back with Li-Ion etc.
Despite their many drawbacks it seems some of these (old) Li-
Ion cells (e.g. the SAFT cells used by Lange) will last ~20
years.
While the cost of an electric launch looks economic compared
to a tug the real costs are much higher......I seriously doubt
that overall the costs are lower; you pay for autonomy!
For FES/Jet sustainer systems (the original post) I'd say the
critical factor is start reliability; sitting in a field with an engine
that did not start must rank as bl**dy frustrating.