There's a very pertinent historical reference point: Ferdinand de Lesseps. He built the Suez Canal from 1859 to 1869, a task which was considered by many to be impossible or at least impracticable [1]. After achieving resounding success, he then turned to Panama to try building the Panama Canal. In this venture, he turned out to have several very wrong ideas (it's not feasible to build a sea-level canal in Panama), and the French effort failed so hard it actually brought down the French government [2]. He was successful in the first venture but for different reasons than initially assumed (namely, the Suez Canal isn't all that challenging from an engineering perspective; it's just capital-intensive), and his failure in the second venture comes from misunderstanding the problems.

[1] There had been canals in the past that traveled from the Red Sea to the Nile delta, but these invariably silted up rather quickly (the Nile was sediment-heavy, and wind-blown sand is quite common in the region). Thus there was historical evidence favoring the idea that the dredging maintenance fees would exceed the revenue from passing ships.

[2] Of course, this was the Third Republic, which is notorious for having provided series of weak, unstable governments replete with scandals that prompt new governments. Ironically, the Third Republic was the longest-lasting form of government France has had since the Ancien Régime.

To this day, the Third Republic is still the longest lasting.

Yes, but we could see that batteries needed to get X better in order for us to achieve Y results in the future. But how much better do vacuum chamber technologies have to get before you can make the biggest one ever built, that also has a high-speed capsule traveling through it, that is also impervious to armor piercing bullets? Furthermore, imagine if every airplane had to fly the exactly same path, and if any failure on that path prevented (at the very least) all subsequent planes from traveling that path. And finally, how are security lines going to get shorter if the stakes will be at least as high as airplanes?

I don't know the answers to these questions, but what I do know is:

1. Elon Musk has thought about them more than we have.

2. If his initial concept turns out to be incorrect in some way, he will change it until it is workable.

Are airliners impervious to armor piercing bullets? As I understand it, a small bullet penetrating the loop will cause a minor release, not a catastrophic event. It's not correct to hold the Hyperloop idea to standards that we don't apply to other forms of travel.

This is the real insight.