* Payments cannot be reversed
* Users are anonymous
Advantages.
As I wrote, bitcoiners call them advantages, but for the other 99.9% they are fatal flaws. A system that does not allow mistakes and crimes to be corrected is a stupid defective system. Anonymous accounts are much more useful to criminals than to honest people. The two together prevents enforcement of property rights.
* Its design cannot support 1 billion users, maybe not even 10 million, even indirectly.
That's actually just not true. There's good reason to believe it could support 1 billion users, indirectly at least.
1 billion users doing on average 1 payment per day would be 1 billion tx/day.
Suppose that most of the traffic gets pushed off the blockchain, e.g. to Lightning Network; even so, there will be a need for blockchain transactions. Let's say that 25 transactions offchain for each transaction on the blockchain. Then the bitcoin network would have to handle 40 million transactions per day.
The current capacity of the network is 300'000 transactions per day. You don't want to operate near capacity even at peak hours, so 200'000 tx/day will probably be the limit with 1 MB blocks. It would have to grow by a factor of 200.
That does not seem bad, but the Ligntning Network is still only a fuzzy dream. No one knows whether it is technically and economically viable, whether people will want to use it, whether it will really achieve 25:1 offchain/onchain ratio.
Note also that, in the 5 years since the 1MB limit was set, the capacity of the network has increased 0%. How long will it take for it to increase 200x?