almost 90% of bitcointalk users are inactive on forums in 2022, will they ever be active again?
or they completely quit the forum.
This metric (true or not) doesn't really say anything. Number of accounts doesn't represent number of actual users. In the "pre-merit" era, account farming was a big thing. And it's quite normal that a lot of people would just quickly create an account only for one-time use purpose and never come back, this is not unique to bitcointalk.
maybe people who are active on the forum in 2018 are just following along with what's trending, they just want to know what's interesting in this forum, and after they get bored, then slowly stop again from the forum.
Don't take it seriously, I'm just assuming.
That's also a normal cycle, very few people would have consistent level of activity throughout their lives. People join, leave, come back again etc. Normal thing.
Although total posts are greatly decreased after 2018, but I'm just curious does it's really good for the forum?
Not really, iirc the level of spam was becoming unbearable. That's what you get when when you provide incentive to posting (bounty campaigns etc) without any quality expectations. It was not uncommon to see i.e. someone just copying someone else's reply from the middle of the thread and posting as their own. Many such cases.
As mentioned above, ICO dying natural death helped the most, I would imagine implementing the merit system played a good part, but it was introduced at the
beginning of 2018 so if it had any effect, it was delayed in time.