I was really disappointed when the iF20 started to dwindle in Europe.
As Jake said, F18 was pushed fowards by manufacturers that developed the right boats to go head to head and even some guys making them in their own sheds.
iF20 became doninated by the inter20 [Not many variants either Rolf] and the other boats that were about were not developed to beat them. the yank habit of tinkering with the rigs to suit themselves did not help either!
There were a lot of F18 guys that were just too big for their boats at the time and we tried to get them to step up to the iF20. When they sailed the inter20 they said that it just felt too easy to sail and not fiery enough!
despite its many knockers, F18 is rightfully the dominant class and it WORKS - you won't see any F16 or F20 boats winning any big races on handicap.
There is only room for one formula class and F18 is it!
The 20 footers best chance is if it sticks to an absolute one design and the F18 gets too pricey to keep competitive [olympic involvement?] making the 20 affordable top level racing.

Paul