How to respond a reviewer's reply when he is asking about much more to be discussed?

One thing to make clear at the outset: it is the editor who controls whether your paper is published or not. Your task is to convince the editor, not necessarily the reviewer, that your paper merits publication.

Most editors will not blindly require you to implement all the reviewer's suggestions to get the paper published. After all, it's not like the reviewer necessarily knows better than you do what is going to make your paper publishable. (If multiple reviewers all make the same suggestion, that's a different story and an editor will put more weight on that.)

If you have a good reason not to do something suggested by a reviewer, you can not do it and point out in your response to the editor why you made that decision. The important thing is to make it clear that you didn't simply decide to ignore part of the reviewer's report. If you're going to avoid implementing any of their recommendations, you do need to justify it. It then falls to the editor to decide whether the paper is still publishable without that particular suggestion being implemented.

I'd add that saying that an addition to the paper would be too large and would justify an entire followup study on its own is a valid response. Even a reasonably common one, I would think. Reviewers haven't done the research themselves; they don't know how much work an extension would take, and if you say it would be too involved, there's a pretty good chance the editor believes that you know better than the reviewer on that point.

What's sometimes worked for me is to e-mail the editor noting the conflicting demands of the referees and saying what you propose to do and asking if that would be OK? Depending on the circumstances I have also asked if doing X is a requirement for publication, and I have withdrawn or resubmitted depending on the response.