I think just in the case of the bitcoin cash split, the fork probably has some value.
After all, it's a split from the dominant currency.
Bitcoin has become a brand and it has public notification.
A fork from bitcoin inherits in my eyes some of the brand value of bitcoin.
At the same time the brand value of bitcoin stays the same.
If the value of bitcoin cash realy is as high as it was/is of course there are different opinions for that.