Well, yes and no. Obviously one site only gets so far for SEO links, but if a site is popular enough then it is very worthwhile to have links from it. My company runs a number of websites, PageRank 4-6, and we have had multiple requests from SEO companies to buy links on our sites (which we obviously turned down).

If they all created one site and posted links from it, they would gain very little as Google would a.) detect that it is a site full of SEO spam and b.) realise that no-one actually reads the site, so the value of its links would be very low anyway.

However, if your site becomes popular, which Google will judge based on a number of things, such as other sites linking to you, then your Google PageRank will go up. With a high PageRank, suddenly you linking out to other sites will become useful for SEO purposes.

There are already plenty of SEO copywriters whose job it is to find popular websites and offer those websites pieces of content in return for a specific text link. They could see your site and start using it regularly, essentially turning it into an SEO content farm, generating SEO value not for you, but for all the websites these SEO people are working for.

"This issue however has been tackled by hundreds of other sites who also have similar issues, like bots creating accounts and posting comments with links."

Yeah, that problem is, for the most part, a non-issue, it's not hard to eradicate that, 99% automatically and 1% with human moderation. But actual SEO copywriting is very real, I have a few friends who are paid to write web content purely for SEO purposes. They could go and get a job creating content for one site, their writing isn't terrible, but instead they look for a site, they write content for it in return for a link - one person might write an article on gaming, an article on software development, an article on politics and an article on medicine all on the same day.

And that's the kind of problem I can see, it would happen like this:

1) Site becomes popular

2) SEO people see that it is popular, and that therefore Google will care about links from your site to other sites

3) SEO people start writing content for your site, which includes their links

4) The majority of your content is no longer really user generated. It still is, technically, but all the users generating it are SEO copywriters.

Now, if that happens, there are two possible end results, and I'm not sure which is more likely. The first is a possible good result - the SEO writers get the links they want, they give you good content, your readers enjoy consuming that content, and everyone is happy. But the other possibility is either than Google/etc. start to see your site as a content farm and punish you in their rankings (which could lead to the SEO copywriters disappearing and your content drying up, as you also lose traffic from readers) or search enginges don't care, but readers stop visiting your site because the SEO content isn't good enough - that would lead to less visits, less links to your site, and the same negative result.

So, conclusion, not saying you definitely have a problem, and if it did happen, it might not end badly for you. But certainly something to think about. While I wish you all the luck in the world, I'm also slightly hoping this happens because I'm now very interested in what the end result actually would be.