There’s not a single downside to requiring all material used in training to be licensed.
It destroys the open source/hobbyist sector. The only AIs that would be available for artists to use would be corporate-controlled, paywalled, and filtered. That's a pretty huge downside.
And they can allow their art to be used to train AI. It just shouldn’t come at the expense of everyone else who wants to do things the traditional way.
Why should my work be open for anyone to use to train AI? I don’t care if it’s a hobbyist or an open model or google. I don’t want them using my work for training their models. Artists currently have rights over their work being used commercially, and I expect AI arguments to go that way as well. If it is to be used then it must be with the permission of the creator and a licensing contract written out. Art can be shared license free or AI permissive licenses, but would not be required to be.