The below answer was originally written in response to this first revision that provided no reason to implement this whatsoever. There has since been a reason added.
In response to that, I have to add that as a moderator on two sites I see and clean up enough comments that I still would actively recommend against implementing this feature. Almost none of the comments I see on answers are requests to clarify or improve the question the answer is answering, so the original points below about unactionable noise still stand in my opinion.
First off, take a look at this post. You have a history of writing feature requests without accurately being able to argue the benefits/downsides of them, at least not in any more depth than 'I would like this and use it if it were there'. In order to write a good request for change, you have to understand the reasoning for why things are the way they are in the first place, and then you can work from there. The biggest thing you seem to miss here, is what notifications are supposed to be used for:
We send people notifications when there's something actionable for them, or when an event occurs that would reinforce their motivation to participate.
Even the introduction of the follow feature mentions something in that direction:
We hope that this new feature will allow you to have better access to the content that you care about and want to keep tabs on.
The follow feature isn't there to keep track of every comment on every answer to your questions automatically, the whole idea is that you can pick which posts you want to follow, so that you can still get useful notifications, things you can act on or that would encourage you to engage with the post.
A feature like you're suggesting could quickly escalate into being nothing more than noise, especially once people start posting answers that you don't care about. If you have turned on the option when there were 2 answers, you're probably going to have to turn it off again once there are 5, including 3 you don't care about. This feature would just move the problem around: Instead of opting in to get updates on posts you do care about, you now have to opt-out whenever a post you don't care about is creating noisy notifications.
Since opt-in avoids the frustration of having to see noisy notifications in the first place, this is in my opinion definitely the better solution.