I'm having a real hard time parsing that paragraph. Putting aside taxation, only two classes of people receive some share of income from production: workers & owners of capital. I don't understand how the income share could be declining for both classes simultaneously. Where is the 'missing' income going?
And bringing in the idea of winner takes all market structures just seems like a non-sequitur. I'm struggling to understand the causal relationship between market structure and the relative shares of income that go to labour and capital owners.