They're replacing Linux with another open source OS so they can make it closed source? Riiight.

I think the main reason is so they can provide a stable driver ABI so that it becomes possible to update Android.

They're probably also frustrated with Linux's security (hence this post) and having to maintain out of tree patches (e.g. Binder).

Obviously this is a reason otherwise Fuchsia would have the same licence. However Google probably doesn't care that much about having an option to keep it closed source. Antagonism to the GPL comes mainly from Hardware vendors.

They cannot make anything of Fuchsia, which has been released under a permissive license retroactively open source. If Fuchsia replaces Linux on a large scale, they cannot take that away. What Google could do is having a closed source branch or release custom builds of Fuchsia, which have closed source in it. While there are certainly some szenarios which could be worrysome, it depends on the open source community on how much Google can get away with that. If only the open source parts are used, closed source version would just not have much meaning.

And of course, there would be some very legit reasons not to use GPL - like embedding ZFS into the kernel. Which would be no license issue for Fuchsia.