So why cant we get a boat,quite like the Hobie 18, that would be a reasonable weight, say 340-390 lbs, and durrable.

If you can make a boat weight 160 lbs and is realatively fragile compared to the ol Hobie 18. Why not use some of that tech savy and build a boat that is 150-200 lbs heavyier and build to be a real beach cat wth spin and all the nice features.
18', 380lbs and built tough.

Then we would have a boat to last. And proovide some enjoyment.

I think durrable glass beats plastic any day. (Hobie 16/18 proved that).