I think all of the users here have used a site that fell into a slow or sudden decline, whether it be Reddit, Voat or something else.
In the case of Reddit, it has many problems. Money has been a big issue. The push to remove and censor certain subjects/content from the site in order to appeal to advertisers ended up alienating a lot of users who just wanted somewhere to post/look at stuff and not have it taken down. Going way way back, you could say it started with /r/jailbait, which (I think) was the first sub to be banned. It was definitely the first high profile sub to be banned. The admins were not giving mods the tools they needed, mods were taking things down even though there was nothing wrong with the posts at all and subs started being banned left right and centre. However, Reddit is still around and it seems to be doing just fine, nothing more than a blip on the radar.
Voat benefited directly from the shit happening on Reddit more than once. It started to be linked more and more as more subreddits were banned, more users were banned, Victoria was fired, the blackout happened, Ellen Pao became synonymous with Adolf Hitler etc. This had the effect of bringing everyone who was not wanted on Reddit over to Voat, making it an anti-echo chamber in relation to Reddit. Voat also has a money issue - they aren't really getting any. Another anti-attribute that can be applied to Voat is they are very very very cautious when it comes to banning things or removing things, because they know that censorship is killing Reddit. Unfortunately, this has led to shit content reaching the frontpage every day, a spam problem and the formation of protectvoat, which is basically the same as /r/ShitRedditSays but they claim to promote free speech on the site.
So what can be done here? Is it a roll of the dice to see if we end up with the same echo chamber that exists on Reddit or Voat? Which mistakes do we avoid and how can they be avoided?
The issue here is that people like to implicitly assume there is the possibility of a site that doesn't suck in some way. I hate to break it to you, but the problem isn't the site, it's the users. Human nature isn't the best irl, and when you add psudo-anonymity to the mix, there's almost no consiquences for making things worse. The only way to have a truly good aggregator site is for it to have few enough users that everyone can police everyone else, or to be curated by one specific person who controls everything. The trick is to accept that all online communities with more than a couple people suck in some way, and live with it.