The fact that they continue to release more stuff over and over gives very legitimate credence to the notion they are hiding something.
It seems clear they don't want us to know something.
Now, it could be more or less political, or personal, or something ultimately mundane, but the conspiracy theorists can make it anything they want of it.
I suggest there might have been something really complicated wherein nobody could point the finger fully at any group, but that the CIA was doing something sketchy, funding the wrong people, Johnson new about it but only in superficial terms, someone somewhere took it too far, it got out of hand and that the truth is just too damaging to the state (and various agencies) itself so they want to keep it hidden.
If an innocent person held some information in their literal pocket, that they could very simply reveal to the world, at no cost to themselves, which proved their innocence of some crime unambiguously - but refused to do so and instead chose to stand trial for some offence - it would be entirely rational to be suspect.
This is why in some cases 'pleading the 5th' can be considered as evidence towards guilt.
The government is more than aware of the vast conspiracies surrounding the incident, not only do they have a responsibility towards transparency in any event, it's even more important in this case.
There's a 100% chance they are hiding something - otherwise they wouldn't hide it.
So the question boils down to only what they are hiding: mundane administrative information? or 'The CIA did it!'? I suggest it's more towards the former than the later, but not all the way. I'm put it at 90% chance there were at least some shenanigans, but I don't think it was as bad as 'the CIA ordered his death' kind of thing. But there were shenanigans.
I think the point is rather: if they keep releasing stuff, it means they held stuff back the last time they released more stuff. It's natural to assume there is even more stuff still being held back.
It's not even that we're "baselessly" (God, the news has made me hate that term) assuming that there's more documents that they're hiding.
Many of the documents they've given us are 50% redacted. There's so many redactions it's almost hard to follow the content of said documents. They managed to release something while also releasing nothing.
It seems clear they don't want us to know something.
Now, it could be more or less political, or personal, or something ultimately mundane, but the conspiracy theorists can make it anything they want of it.
I suggest there might have been something really complicated wherein nobody could point the finger fully at any group, but that the CIA was doing something sketchy, funding the wrong people, Johnson new about it but only in superficial terms, someone somewhere took it too far, it got out of hand and that the truth is just too damaging to the state (and various agencies) itself so they want to keep it hidden.