The demand signals never allocated the resources, a small number of people always did. Those people react to the demand signals (and they still do).

IMO a more important question is: what happens when the market power of a segment of society grows much greater than the rest? In a "K-shaped" economy, the top 20% have so much spending power (and even more future spending power) than their demand signals can overwhelm the rest. Then IMO the market won't respond to the demand signals of the bottom 20% and capitalism stops working for all. Add to that the possibility of AI automating more and more work and you have a system that only works for folks at the tops.

This isn't a resource allocation problem! it's a purchasing power distribution problem.