1,5 mil. is a performance bonus, the sail are lighter, durability is the key factor: kleenex sails are not in F18 way.
Andrew write that 1,5 mil. = 3 mil. for durability.
I ask him to prove it.

Using 1,5 mil. material is cheaper (Alex Udin write that), but his sails (using 1,5 mil. ?) are as expensive as others...

The key point is to have very close F18 on the start line, thin sails like paint on the hull are true performance bonus.
No sport interest to have a F18 faster than the one next to you on the starting line.
The genuine purpose of F18 is to get as similar boats as possible.

If the rule process goes to its target, grandfathering would be ok, no problem about that, I think.