Lack of novelty is a general killer and I couldn't hate it more.
It all depends on the journal and editor, you may still have a chance if the rebuttal letter is well written and sound eloquent from every aspect.
Here is something I would do, can't guarantee it will work
Start something with,
We thank the reviewer for the suggestion. While we agree with the reviewer
that addtional XXX studies would enhance the significance, the primary focus
of this study is to XXXX (put your emphasis here). In this vein, such
studies are beyond the scope of this study and we are currently
investigating (XXXX), which will be reported separately.
I am writing a rebuttal letter right now:) Lucky this time, I didn't get
such comments.
Good luck!