I don't know how this specific thing works, but I don't really see any fundamental problem with mixing and matching. If you believe in the benefits of crypto, then 50% crypto is still possibly better than 0%.

It's not like I forgo a lock on my front door just because my windows are made of glass.

Currency isn't a homebrew computer or backyard car project; it is either centralised or not; there is no in between.

Blockchain with central authority is the worst of both worlds.

Not really. At a traditional bank I have to trust n people with varying degrees of access. Et ceteris paribus, any reduction in n is an improvement, even if n is not zero.

Of course n can be smaller and the specific people less trustworthy, but that's quite a different thing.