Who says it isn't mature? Is it immature because it doesn't fit your content expectations?
Reminder that after 15 long years of chaos, it still exists. It's a very specific, essentially impenetrable market of 4chan users that can't really be pushed away except by catastrophic mismanagement. The features of the site have stabilized and reacted to user demands over the years, and though good boards (/prog/ :( and others) have been terminated, lovely discussion still happens.
4chan isn't selling your identity to data scientists. 4chan doesnt fail whale every few moments, 4chan doesn't have a huge amount of bot accounts relative to human users, though I'm sure there are some.
People love to hate 4chan because there are deplorable users that use it, but it's just disingenuous and dishonest to make the claim that 4chan isn't a good product in terms of capturing their desired users.
The fact that so many other chans exist is a testament to the success of the format. Reddit has an entire sub reddit /r/RedditAlternatives, which I think accomplishes the same thing: letting reddit know "the product is good, I just don't like YOU."
The difference is that 4chan is a very strong anchor of all chan-related traffic, outside of Japan and Germany. "competitors" don't really pose an existential threat, but rather a topical, and therefore fallible threat of feature envy or fashionable content envy.
Disclaimer: I haven't used 4ch since the /prog/ days, but I check in now and then, and lo and behold, it still exists.