Why don't we already have a permissionless payment system that runs on the internet? Or a global, digital store of value with a fixed supply? Well, the simple answer is that we would stop ourselves from doing it. We aren't limited by technology, or electricity, or intellect. We stop ourselves. If a company were to spring up that allowed any two people on Earth to send money to each other, governments would quickly try to regulate it, and it wouldn't exist for very long.
Bitcoin requires a prohibitively large amount of computing power and electricity to attack. We had to build it this way. It's like we couldn't help ourselves or something. We needed something that even we couldn't stop if we wanted to.