Will development or maintenance of them cease? Will vast swathes of code have to be rewritten as those modules become unmaintained?
That's a question for the current maintainers of all these modules to answer, and if they don't want to continue maintaining their modules, to the user base. Modules have become unmaintained for a variety of reasons, and "it's now in the core" is not the most frequent one. If no volunteer is found to step in, then it will still take some years until users must rewrite their code since Perl will always value compatibility.
And also, it's a loooong way. The first version of Corinna lacks a lot of features which have been added to e.g. Moose or Moo.
Either way I hope the decision to include any such system in the language itself is not one which we shall come to regret down the line.
We might regret it if we have the expectation that someone else will, without us paying for that, maintain the modules we want forever. But I hope that we are not like that, otherwise I'd say: we deserve no better.