Well, what about #2? “All the information in the message is in the public domain anyway! There’s nothing new in this message that can’t be discovered on the blockchain.” That’s almost correct: being able to watch messages fly around allows you to attribute a message to the source a lot easier. This isn’t an issue for most people, but if you’re performing huge transactions from your home office, someone might be interested to know where that home office is located as it’s more than likely your private keys are there too.
The most important fact that counters #1 and #2 is this: your postcard will be handed between a number of people, all of whom have the ability to read and modify the content of the message before it reaches you. Modify, change, edit. Let’s say that you send a postcard to your friend, asking them to send a payment to your bank account ‘123456’. Your not-so-friendly mail courrier changes your message to include their account number, ‘654321’ instead of yours. When your friend makes the payment, they will now be paying the wrong person so you won’t receive your money. By the time you realise what’s happened, your postman is off on holiday with his new fortune.
HTTPS, in a very gross oversimplification, would be the equivalent of putting the postcard in a lockbox to which only the recipient has the key. And any responses from the server would arrive to you in a lockbox that only you can open. Now, a powerful law enforcement agency could take a shot at trying to get in there and maybe succeed, but unless you’re a modern day Pablo Escobar trying to run your smuggling operations on blockchain, you’re probably safe.