1. Even if there's no cash-out people are going to try to find a way
Your online economy might be specifically designed to prevent people cashing-out, and turning their online profits into actual profits. You've been careful. You've designed everything just so. Unfortunately, that isn't going to help much. The stainless-steel rats will work at every crack and crevice to find a way to turn your virtual currency and items into legal tender. Once they've found a crack, they'll widen it as far as possible.
Look at World of Warcraft. The trade in gold, items, and powerlevelling is far beyond the ability of Blizzard to cope with it. Even if they wanted to, there's just no way to stuff that genie back in the bottle.
2. If there's a cost, people are going to try to reduce or eliminate it
Whatever the cost (in virtual currency, dollars or time) and however small that cost might be, your users are going to try to make that cost smaller. If they can find a way to eliminate it entirely, so much the better. Botting is one way of reducing costs. Item/gold duping or ripping off other users are others. If there's a cost barrier, people are going to try to batter it down, go around it or tunnel under it. Anything to prevent going over it.
With that in mind,
3. People are going to steal from each-other
In economies where people can create their own stuff (such as Second Life, to name one example), one of the simplest ways to shortcut costs is to simply copy someone else's work. It's a really low thing to do, but it is unfortunately common, and can be very hard to programmatically detect. Absurdly simple transforms can eliminate detections of texture watermarks, and in many cases a texture can simply be flipped, mirrored or offset to avoid automatic detection. Meshes can be subtly altered or rescaled. All of these can be done without any specialist technical knowledge or skills.
People shouldn't rip-off each-other's work. Unfortunately they do. More than once, two users will complain that the other is ripping-off their work, only to have it revealed later that they're both ripping off a third-person, or that they both sourced (pinched) a texture or a mesh from the same Web-page, magazine, or video game.
In economies where people can't create their own stuff, some people just resort to theft of one sort or another, whether that's confidence tricks, phishing or some other form of account-hacking.
4. Coincidences happen
With tempers running high about the possibilities of the sorts of content copying above, unfortunately coincidences do happen. Two content creators who have little or no knowledge of each-other will sometimes make the same basic thing. Sometimes it is almost identical. Everyone cries foul, but nobody is really to blame. It's hard to distinguish this from actual infringement. It's a mess, but it happens.
5. If you try to patch an online economy, you could collapse it
We're not just talking about fixing a gold/item duping bug, or fixing a simple permissions glitch. The older your economy is, the more fragile it becomes in the face of policy changes that change the fundamentals. You'd think that time brings a sort of diversity and richness to an online economy that makes it stronger.
You'd be wrong. Just like AFK economies, online economies flow into channels of least-resistance. Lowest-costs, highest-profits. Economic activity increasingly moves away from less profitable activities and gathers in more profitable channels.
Anything that significantly disrupts or changes the costs or profit-margins associated with those channels is like an economic earthquake, whose effects are hard to predict. Maybe only a few houses collapse, and maybe there's widespread ruin. Economic activity will try to find and establish new channels, but the harder those channels are to find in the wake of the disruption, the less likely the overall economy is to survive. AFK economics has certainly taught us all about that over the years.
This can be especially harsh if the existing channels are already filled to capacity with other users and businesses.
If you think all of these things seem like they might apply equally well to your own AFK economy, you'd be right. Whether your economy runs on US dollars, gold, coins, Linden Dollars or vespene gas, it's the people that give the tokens meaning, create the markets, and generate the activities and business-models that take advantage of them.
There's very little practical difference between AFK economies and online economies despite the best efforts of the people who create the online economies – because they're still accessed by people, and people will always try to bend an economy to their will.