There are essentially three big problems:
1. Resource depletion.
2. Lack of scarcity.
3. Non-capital economy.
Resource depletion reveals a very simple and horrifying fault in capitalism: If you sell all the shit, you will no longer have shit to sell. Everybody starts killing each other out of desperation. This was not a serious problem back when Adam Smith wrote his thoughts on the matter, sadly, pretty much nobody saw this current order coming. Ultimately, no matter how much they consume, 1 billion humans just aren't going to deplete Earth. But when 1 billion humans inflates to 7 billion humans in under two centuries, shit is going to go down. Capitalism encourages maximum resource extraction and only rarely encourages resource investment, and as such is no longer viable when we're going to soon start to have using what we've got intelligently.
Lack of scarcity is the opposite issue, on things that capitalism has "succeeded" on. This success can not be complete, however, as that would render the capitalism irrelevant. Capitalism will never make that final leap to post-scarce materials, because it has no profits. Eventually certain capitalists who shall remain unnamed will try to oppressively create enforced scarcity in order to return the market to its "sweet spot". This will create an ever-growing rage towards the scarcity creators among the population. You just can't sell things if they aren't scarce. That's simply how it works. Why buy what you can get for free, or essentially free?
Non-capital economy is what I think is the true nail in the coffin, though. Things losing scarcity can at least theoretically be sold, non-capital doesn't function period. You can try to make it function, and with sufficiently draconian measures some non-capital can be hammered into a capital-like shape (here's looking at you, RIAA), but in the end it just doesn't work and pisses people off. The internet and anything that can be transferred through it are a huge part of this, and only growing larger as time passes. There are other things, though. How are you going to have capitalism when you can print anything you need that is made of plastic or metal? (Protip: Most things you use these days are plastic and metal, and much of that which isn't only isn't out of convenience). You could sell the raw resources, sure, but that's going to leave the middlemen (see: almost every section of the economy) without any profit. How are you going to have capitalism once your "job creators" can just have machines do everything cheaper than even the most impoverished African street urchin? How are you going to have capitalism when people who don't see any fault in internet piracy run the government? How are you going to have capitalism when you can download a car?