First you have to have the VISA network in the first place, that's no mean feat. Then there are the banks themselves, hardly trivial. Only then do we get into central banks and financial regulators. All part of the package.
Really though, the fact that every single transaction is broadcast and that every single transaction must be verified by the entire network (in theory) is what kills Bitcoin's energy efficiency. Whenever a new mining rig is brought online, the energy efficiency of Bitcoin decreases -- and Bitcoin is designed to incentivize bringing more mining equipment online.
It is also the fact that ensures it's distributed nature, and the purpose of its very existence in the first place (we have plenty of centralized digital currencies).
> Whenever a new mining rig is brought online, the energy efficiency of Bitcoin decreases -- and Bitcoin is designed to incentivize bringing more mining equipment online.
This is not necessarily true. The efficiency (number of SHA256 operations per joule expanded) will increase over time. It has already increased by a factor of 1000 compared to CPU mining. So it's definitely possible for both the security and the energy efficiency to increase simultaneously.
My concern about bitcoin winding up more expensive is that it's a bit of a red queen's race. Unlike most applications of encryption, attackers and defenders are on even footing (except, like in most human spheres, in that defenders hopefully outnumber any coordinated group of attackers). This means that whatever latest best tech we have for efficiently turning power into hashes, we've still got to burn about the same amount of power to keep things secure (and continually update to the next newest tech). In other spheres, technological process makes doing the same work cheaper - but in bitcoin it stays the same; you've got to run as fast as you can just to stay in the same place.
It may well still be worthwhile, though.
The growth of Bitcoin is nothing but a symptom of the lack of faith most of the world have in the policies of the US Govt, the Fed, their politicians and other Central banks in general. They have proven that they simply can't be trusted.