For me, 20 knots is perfect for:
bump an jump on a 105L board and a 5.7
slalom on an iS111 and a 6.5
formula on a F160 and an 8.5 or 9.2
All are fun & exciting and I do all three in 20 knots, it just depends on how I feel that day.
Chasing keel boats and cats on a formula board, and sailing circles around them upwind and downwind is a kick. You can't do that on a slalom or bump and jump board.
I do what I enjoy, regardless of what others think should or shouldn't be done.