No, they legally can't at that point.

They can still sue for trademark infringement, but it’s more complicated.

I think

How are we supposed to know what a company is going to do in 2034?

Please read this.

I think DC will try to find anything about people's use of the characters to sue them for when they go into the public domain. Like try to find the smallest detail of something that could violate copyright in order to sue them.

Disney Animation started using Steamboat Willie in their logo because they could trademark that and further limit usage. I expect other companies to use similar tricks.