The existence of zero marginal costs are not a problem for capitalism whatsoever. You're thinking in a very inflexible way. You're trying to take the model of economic activity today and map it with only a few adjustments onto some future world. But this is as foolish as trying to model the economy of 2015 from the example of the economies of 1950, 1850, or 1500.
I think the economics of air is an exemplary point here, worthy of delving deeper into. Throughout human history air has not been a subject of economic activity, precisely because it is free and abundant (and I'm setting aside the discussion of air pollution because it's a more complex subject and not strictly relevant). But despite the fact that air has always been a critical necessity for all human activity, the fact that it has not been the subject of economic activity has not been a hamper to other economic activity, in fact if anything it has been a boon. Internal combustion engines benefit greatly from the free availability of air, for example, to great financial and functional benefit to many.
But what happens in the context of, say, a commercially run colony on Mars? There air is very much not free and must be paid for in some way, either through fees, taxation, or metered use of some sort. But even then that wouldn't change the economic foundation of society in, say, rendering "capitalism" obsolete or not obsolete.
The point being, that what is and is not the subject of economic activity can change, and change drastically, between different contexts, but that doesn't require a grand revision of economic theory. In the future a lot of things that are currently very expensive may become extremely inexpensive or free. But this has happened repeatedly throughout history. What happens is that economic activity moves elsewhere. Today getting enough food to survive is very easy to do, even at a minimum wage. This is very unusual historically. And lots of other things were once tremendously expensive, such as: trans-continental travel, world-wide messaging, books, personal transportation, clothing, durable high quality silverware, and, of course, spices and "exotic" or "rare" fruits such as watermelons and pineapples. At one point pineapples were so exotic, expensive, and sought after that ship captains used to use pineapple motifs for decoration as a status/wealth indicator. Today, of course, you can go to any grocery store in any city in the US and buy one for a few dollars, or get canned pineapple for even cheaper. And so what? All that means is that people aren't working in order to buy pineapples, their working to buy other things.
The economy of 2050 is likely to be very different from today's, but it will still have a lot of familiar characteristics. People will still go to work, people will still exchange goods and services with one another. There will still be a huge amount of economic activity, though it's likely that many of the things we consider valuable today will be nearly valueless then and many things we are not even aware of today are considered valuable.
I think you are handwaving the issue away with »economic activity moves elsewhere«. Now that I have said this I have to handwave myself because I am not sure how to quantify this. I believe there is a much harder limit to the amount of things you can reasonably consume than to the things you can produce. 2500 calories and 3 liters water per day, one house, one car, one phone. On the other hand nothing really limits the amount of things you can produce if you are supported by enough machines.
The obvious consequence is that there is not enough work for everyone and therefore working for money and spending money for goods and services no longer works. And I am pretty sure we are already there in some sectors. We are already producing and throwing away hundreds of million of tons of food every year. We are already replacing mostly okay phones every year or two. Or look at these bullshit jobs, people generating reports all day long and handing them over to the other guy shredding reports all day long. What would the unemployment rate be if we removed all those inefficiencies and all the waste?
I think the economics of air is an exemplary point here, worthy of delving deeper into. Throughout human history air has not been a subject of economic activity, precisely because it is free and abundant (and I'm setting aside the discussion of air pollution because it's a more complex subject and not strictly relevant). But despite the fact that air has always been a critical necessity for all human activity, the fact that it has not been the subject of economic activity has not been a hamper to other economic activity, in fact if anything it has been a boon. Internal combustion engines benefit greatly from the free availability of air, for example, to great financial and functional benefit to many.
But what happens in the context of, say, a commercially run colony on Mars? There air is very much not free and must be paid for in some way, either through fees, taxation, or metered use of some sort. But even then that wouldn't change the economic foundation of society in, say, rendering "capitalism" obsolete or not obsolete.
The point being, that what is and is not the subject of economic activity can change, and change drastically, between different contexts, but that doesn't require a grand revision of economic theory. In the future a lot of things that are currently very expensive may become extremely inexpensive or free. But this has happened repeatedly throughout history. What happens is that economic activity moves elsewhere. Today getting enough food to survive is very easy to do, even at a minimum wage. This is very unusual historically. And lots of other things were once tremendously expensive, such as: trans-continental travel, world-wide messaging, books, personal transportation, clothing, durable high quality silverware, and, of course, spices and "exotic" or "rare" fruits such as watermelons and pineapples. At one point pineapples were so exotic, expensive, and sought after that ship captains used to use pineapple motifs for decoration as a status/wealth indicator. Today, of course, you can go to any grocery store in any city in the US and buy one for a few dollars, or get canned pineapple for even cheaper. And so what? All that means is that people aren't working in order to buy pineapples, their working to buy other things.
The economy of 2050 is likely to be very different from today's, but it will still have a lot of familiar characteristics. People will still go to work, people will still exchange goods and services with one another. There will still be a huge amount of economic activity, though it's likely that many of the things we consider valuable today will be nearly valueless then and many things we are not even aware of today are considered valuable.