Why keep this whole structure of central banks, commercial banks, and all this BS, if we can just use crypto *directly* ?
There are lots of reasons why crypto - at least in its present form - cannot be used "directly". Some are economic and others are simply practical.
For a start, an SQL database server out performs any blockchain based transaction system by many orders of magnitude. Blockchains were not designed for supporting high volume commercial trading interfaces but for manifesting a basis for a peer-to-peer token which would serve as a store of value. (For example, all trading that goes on in exchanges is done in a database off blockchain, otherwise it would take forever. The same would go for supermarkets if they ever denominated the price of cabbages in BTC).
I agree that cryptocurrencies as designed today have a fundamental problem with very high volumes. Bitcoin has a hard limit, others, like monero, can adapt, but at a certain point, the block chain growth becomes so large, that with present technology, this is problematic. Maybe a 500 TB block chain is not a problem in the future ; for the moment, it is.
That said, there could be solutions to that. Off-chain is one answer. But "grouping transactions together" could be another one. Suppose that a supermarket receives 300 transactions per minute from its customers. There could be a kind of cryptographic technique, style zcash, that allows this supermarket to combine these 300 transactions into one single big one. I don't know of any block chain that implements this at the moment (because no need: the only block chain that has problematic high volume at the moment is bitcoin, and there, it is hitting a hard protocol limit) ; but if so, it would again be something like an "obfuscated cryptographic proof that these transactions were OK without showing them".
Honestly, if the idea is NOT that crypto goes more or less mainstream in the long run, I fail to see the point of it.
The question of whether we use crypto directly or not is really one of price denomination rather than transaction technology since the commercial realm does not care how prices are denominated and can transact in any configured currency. That will be case whether Bitcoin or any other crypto sweeps the world as the universal denomination of value or not.
It is a chicken-and-egg problem. When a currency is really used as a denominator, a property, called "stickiness of prices" occurs, which calms the price fluctuations and stabilizes the currency value. But people will only accept denominations in a crypto when they think it is stable enough.
Secondly, you've got to take into consideration that - notwithstanding the corrupt practices in the fiat system that lead to the current debt bubble - we're not living in the middle ages where economies moved at snail's pace compared with today. In a dynamic economy who's size varies from year to year there are only two choices:
A. use a fixed supply currency and experience wild price fluctuation that arbitrarily bankrupts half of industry and allows the other half to make supernormal profits
B. use a variable supply based on liquidity demand to stabilise prices and create a more predictable economic environment for business
Crypto doesn't have to have limited supply. Bitcoin has a limited supply in 130 years. Monero has tail emission. You could easily provide for a slightly inflationary currency. BTW, our currency is NOT stable. We've suffered an inflation of a factor of 23 since one century.
And there's another regulatory inflation: the NUMBER of cryptocurrencies. If you have 300 different cryptocurrencies, each with a fixed supply, and you create 700 more cryptocurrencies, eating in the same store of value market, you have also a form of inflation. If a cryptocurrency hard forks, you also have a form of inflation. So there is an inherent variable money supply, by the number of cryptocurrencies/hard forks and their relative market caps.
The crypto market is much more elastic than one would naively think.