> Payments are the perfect application found, so far, for the blockchain.That statement really needs to be tempered with context.
For some relatively recent blockchains, and for particular tradeoffs that enhance transaction throughput over raw security guarantees, payments are a practical use case, yes. But of course, the parent was talking about Bitcoin, and Bitcoin is a miserable payments protocol.
There really shouldn't be any sentimentality to that second assertion. It's simply a poor currency according to the metrics of price volatility, transaction verification speed and transaction fees. If you redefine the ideal parameters of a payments protocol as trustlessness, permissionlessness and decentralization, then sure, it seems better. But then we're disagreeing on what constitutes a robust payments protocol according to normative statements, not positive ones. I think the only people who would agree Bitcoin constitutes a robust, practical payments protocol are those who really want its qualities in a payments protocol. But in the real world, companies like Stripe have very specifically and publicly abandoned it for their own alternatives.
I do think blockchains can be used as a payments platform, with the caveat that I think proof-of-stake based consensus and various throughput improvements (not lightning network) in other blockchains are the way forward for that use case. Stripe has invested in Stellar, Tendermint/Cosmos can scale up dramatically, etc. I also regard Bitcoin as a cultural landmark of exceptional importance because it inspired many follow-on innovations in research and engineering. However, for most practical applications, I consider Bitcoin to have failed (thus far) in its original goal as a payments protocol, and to be largely superseded in practice by various alternatives.
As you said: Bitcoin isn't designed to be efficient. But if payments were actually the "perfect" application for it, it would actually be efficient, because the utility of a currency becomes higher the more efficient it becomes (e.g. higher liquidity, higher stability, etc). As for blockchains in general, I don't think payments are really the perfect application for any of them, because I disagree that decentralization, permissionlessness or trustlessness are desirable qualities in a payments platform - each of those reduces efficiency, which leaves you with private blockchains that tradeoff one or more of those security guarantees. They can be used for this purpose, but I don't consider them ideal at the moment.
Rather, I think blockchains map well to solutions for hard cryptographic problems and fault-tolerant distributed systems.