It's easy to limit access to places. The problem is deciding what that looks like, and when they should get access to the next bit. And deciding whether access should have been limited anyway, which I still don't really agree with.
Should people have read-only access to "meat" discussions, or no access at all? If no access, then this will disappear from the public internet and be no longer available as a resource to people browsing the forum looking for information. So that's not an option, it's too big a change.
We'd have to be talking about only limiting people's write-access, but still being able to read it all. Which really defeats the purpose as they're still just as easily scared off by it.
Getting access to the next part can be done automatically (e.g. a certain time after signing up, or after they've made a certain number of posts, or received a certain number of likes, or whatever) - but that's frustrating to everybody as it doesn't really work for anybody, everyone's different.
Or manually (only on approval of a moderator - but I really don't want to add that workload, and it's not effective as people will end up being forgotten and missing out for years while everyone assumes they already have access).
Or with an opt-in system, with people changing something in their own personal information to get access (but that's confusing and people will get very frustrated figuring it out).
Furthermore, people lurk here for months, even years, and then sign up when they want to post something. Or their spouse is here for years, and they're familiar with the forum through them, then sign up themself to post something. If we limit everyone to only the "milk" section (for want of a better word), it just frustrates people who have been lurking, now want to post, sign up - and find they still cannot comment on the one thread that they wanted to comment on. Just because someone is a new signup doesn't mean they're actually new.
I'd prefer to keep things as simple as possible, and not hold people back or confuse them if we don't really need to. Let's find what we can do that achieves the required goals without being quite that drastic.