I'm a big fan of bitcoin and a few altcoins, including some PoS. But I feel like you are understating bitcoin's security. It has been very well tested and has a large bounty - both in its market cap and value of transactions that have not been susceptible to any double-spend attacks.

Other coins might sound good on paper -- adding new features but introducing serious design errors that have yet to be discovered because they're not a big enough target.

You're right. But security today does not guarantee security tomorrow in a landscape where mining incentivizes economies of scale. The more mining power converges (as it is already doing), the more likely for a fewer entities to collude. While it may not be in their interest given their investment in the project, who says that a government or malicious actor might not?

Metcalfe's law is also an interesting way for measuring value of the network

https://en.wikipedia.org/wiki/Metcalfe%27s_law

another is privacy. Bitcoin is not private.

Monero is exclusively private and ZCash is opt-in private.