They don’t need anyone to buy products if they already have all of the resources, and an army of drone soldiers and slaves. For an example, watch Elysium. It’ll be like that, but without the opportunity to revolt.
Hell yeah! People in 50s and even 20s worked 40 hours per week to feed a family of 4! Now we can do that by working much less than… wait, not even 2 working parents safely feed a family of 4? Even with all the gains in productivity?
But more seriously: There is a lot of stuff that AI can't do. And is far off of. It can barely do anything at all for me. Like translate stuff or re-write an E-Mail. The rest is hype. It can't do my laundry, clean up the kitchen. It can't drive the train that gets me to work, not fix a toilet. And it's years if not decades away from being able to do it. I'd be worried if I had some callcenter job or first level support. Or was a useless manager who just pushes paper around all day. These jobs are going to be replaced fast, yes. But it'll take some time for lots of other jobs.
And if we (at some point) advance to a future, where we live in abundance, and technology can do all the hard work, so humans don't have to... Wouldn't that be great? We could do whatever we want. Of course culture and society has to change. We can't have concepts like salaries if we don't work. But by definition we'd have our basic needs met. Food will be enough, or we wouldn't not work anymore. So I guess we just do away with money. Or everyone gets a fixed amount. You could make up your own job. Do arts and crafts, or travel, or spend the day with your kids.
There is one big caveat, however: We won't automatically arrive in a Star Trek post-scarcity utopia. Currently all the AI is owned and designed by big corporations. They also own the computers to run it and they are in control of it. And there is a lot of corporate greed, lobbyism and generally unhealthy divide. I'd say it's very likely that rich people and big, greedy corporations will want to keep everything to themselves. The rich will get richer and assert their dominance with this powerful tool. The poor will get poorer. And can't compete with that at all. And despite theoretically living in a sci-fi world, it'll be a dystopia and end in a big mess / class war / oppression.
But yeah, you're right. Our current form of economy with supply and demand, and money, won't work under those conditions. And I don't think there is a fix to it sou we could keep it.