I can see how you could make this kind of analysis but I think there are a few of issues in how you're conceptualising things. Sorry for the long post, but you seemed to be genuinely asking a question and so I wanted to try and answer it as best I could.

In my opinion, there are two basic problems in your analysis. The first is that you're imagining that there's a largely one-way process in which capitalism determines people's desires/identities, so that as capitalism develops people's identities become more and more fragmentary. I'd argue that this process gets things the wrong way around, and does so in a way that both gives capitalism too much credit as a progressive social force and effaces how society is actually transformed by the radical demands/movements of minorities, workers, etc. For example, corporations have begun to use branding based on the successes of social movements like the queer liberation movement and the civil rights movement to sell products, and the media is increasingly fragmented as new technologies allow for the production and distribution of more and more targeted products. You could argue that this is capitalism creating new niche identities. However, more realistically, what is happening is that capitalism is responding to identities that already exist in various forms and to the success of movements have won certain battles for social recognition and legal rights. It this seeks to incorporate these identities into the circuits of capital - which both opens up new avenues for profit, but also is an attempt to control and shape them in certain ways that are beneficial to capital, just as it does with all identities and all things. Ultimately, though people are really fragmentary just because they are people. Your race, gender, class, sexuality, mental and physical capacities , etc. are all going to intersect in different ways and in ways that create tensions within yourself. Of course, these identities are social constructions, but they are also real. I think perhaps it's useful to try get away from the idea of the self as a coherent whole and to accept that we are all multiple people and work from there.

And I think that this kind of leads into the second problem, that you're implicitly imagining a kind of nature/culture opposition in which the thing that feels natural to you - because you grew up in a monogamous culture - appears to be being changed by capitalism, while overlooking that this thing is itself only a historical and culturally-specific norm. That is not to say that polyamory is anymore 'natural', just that, as I read it, your analysis implicitly codes polyamory as a negative transformation due to seeing it being brought about by contemporary capitalism and the effects of the attention economy. On a less conceptual level, I also think you're kind of misunderstanding what poly relationships are like. It's not just hook-ups, they're basically exactly the same as monogamous relationships, except with more orgies and when you meet someone else that you have a deep connection with, you don't have to make a decision on whether to break up with your current partner. The stuff you describe with attention, etc. is just as applicable to monogamous dating, if not more so, because you have to end one relationship to start another.

In my view, a lot of the contemporary popularity of poly relationships is ultimately the result of the successes of queer liberation, which although it failed to challenge capitalism as an economic system (and was it reasonable/realistic really to ever expect it to do this?!), has been incredibly successful in breaking down the absolute hegemony of hetero-normative social forms. It's not a surprise imo that most of the people I know in poly relationships are queer. However, I do think that there are certain ways of reading the growth of polyamory in terms of the effects of capitalism that avoid an implicitly negative coding. The most obvious one is how unaffordable housing has become and also how difficult just surviving is now unless you're on a really good income. This has had really obvious effects on how people live and how we organise ourselves socially. Many people end up living with housemates until a much later time in their lives than they would have previously, and many people are unable to start families (or just don't see it as desirable). That this opens up a lot more space for thinking about what they want their relationships to look/how they want to live is kind of silver-lining imo