.. What would you all
think about a trailer in which the boat sits on a set of rails and the
rails
slides into the water to launch the boat ?

I can see it working good for lighter boats, but the mechanism and weighting
for heavier boats might present some problems, not to mention the additonal
cost and maintenance of the movable rail setup.
Here's one potential problem:
Many vehicles have the parking brake only on the rear wheels, so if you set
the brake and then use the extender rails to move the boat back the trailer
tongue weight is reduced and may even go negative with a larger boat. If
the tongue weight is reduced enough, then the vehicle might slide backwards
somewhat. Oops.