What credibility? Even on 4chan, which has next to zero credibility AND no voting/reputation system intended for 'self-policing' (so it is not likely to ever become credible), some users express their dissatisfaction with low quality or spammy posts by suggesting the perpetrator go to, or return to, Reddit. This is also a common occurrence here — should a HN user make a useless post, particularly when invoking a meme or trolling, that user is often reminded that HN is not Reddit. This is not a proof of Reddit's lack of credibility, just an interesting/amusing anecdote.

Reddit is not an oft-cited source of news. By and large, it is a marketplace for link sharing coupled with a comment system and a currency of reputation. In other words, a forum focused on reacting to content elsewhere around the web but with some original content here and there. Where there is credible content, it is often from somewhere else like a news vendor. Everything else, including all comments on any thread, must be subjected to scrutiny and distrust as with any other forum. For starters, any comment can be a deliberately hyperbolic or entirely false/nonsensical assertion — rhetoric or sophistry — and thus no comment should be trusted on the grounds that it being correct may be coincidental if the intention was not to be correct, but rather to incite a reaction from others (trolling).

Then there is of course the possibility (indeed inevitability) that posts will be edited silently by those with sufficient permissions in the forum system, or access to the database if the system does not have silent edits built in. I say 'inevitable' because, given enough time, those with access to administrative power or the database itself will find a reason to silently edit something, by someone, somewhere.

No forum should be treated as credible. Even if your study is about how forum users behave, you cannot trust those you study to be behaving normally as their intentions are always questionable. We don't have Asimov's psycho-history yet.

I do not think forums have ever been credible sources of information, and I have participated in discussions on forums for fifteen years now. HN has more credibility than others, but that has been earned by clever people who visit this community for the sake of discussing intellectual topics — not the founding ideal of Reddit.

If the US has had congress hearings on the basis of Reddit posts, as was stated in a comment above, that is testament only to the ignorance of the US congress.

Edit: if you mean viability when you say credibility, i.e. in terms of generating revenue/getting investment, that depends on the rationality of its current/future investors. Assuming rational behaviour, this probably won't make any difference. The users will still come and if anything Reddit users should feel better precisely because the CEO fessed up to silent edits. It means that the issue can be addressed, perhaps with PGP signatures as suggested by others here; the ability to make silent edits by administrators could be removed; and the code powering Reddit could be open-sourced to prove that (apologies if it already is open-source, I am ignorant of the state of Reddit's back-end).