Tell me about public blockchain apps that do not involve objective money or objective identity, which require strict global consensus, why they should be built on public blockchains, and why haven't they been built yet? I'll consider writing a post, when I get back to it, to explain why it does or doesn't make sense

Basically all DePIN apps/networks are meant to crowdsource infrastructure from the public. The application isn't onchain money or identity, but they use onchain money and identity to align incentives. 

What are your thoughts on DePIN?

All examples of DePIN I've seen so far are blatant ponzis