The Italian law for getting your licence was made in such way to give a little edge to the Italian manufacturers.
350 cc, a minimum weight and a maximum horsepower and topspeed made it possible for Ducati to scale down the capacity of the (500-600)engine and still make a bit of money. The RD350 and such japanese bikes were or too light or made too much power to be ridden by the beginning bikers.
Ducati made no priority for the worldwide selling of these bikes, being underpowered and overweight, but wanted to tie customers by creating a steppingstone for later more powerfull Ducatis (hence the F3).
The 400 was created for the Japanese market ( and later some specials too like the Bimota Tesi 400 and the Superlight400).
Nothing screaming riding a 350 Ducati with its redline at 9500 and a powerband that drops before that. Cams limit revving it higher.
I ride my Alazzurras 350 and 650 almost daily but use the 350 more when it rains. Not too much power

ciao
ducadini