The quantity and quality of communications that have been externalized and become effectively searchable and retrievable has increased by many orders of magnitude since paper mail was invented. I think even if you agreed that the State might have had that right then (which not everyone would), it would be good to reconsider what that means with the communications and information systems of today.

They can have the right to surveil, but I don't think they should have the right to demand understanding of my messages.

A term I've seen thrown around is "good old-fashioned police work" and to me the clear distinction is that it can't be done en masse - a real human police officer had to spend time tracking down each known suspect. E.g. clipping a pen register onto someone's telephone wires, tailing someone, asking the hotel clerk if he's been there, unlocking someone's phone to read their messages, and even watching footage on airgapped CCTV systems are all tasks that can be done one-at-a-time and require human power proportional to how much of them you do. This forces the state to only use them on actual suspects instead of using them on everyone, but still doesn't prevent the state from catching criminals.

Even many dictatorship states, like many of the "socialist" countries, had in the past articles in their constitutions that guaranteed the secrecy of the mail and of the telephone communications (other telecommunications did not exist at that time). Thus it is false that there is some kind of established tradition that governments may have access to private communications.

In reality the secret police did not care about laws or constitution, so private mail correspondence was intercepted and opened, read, then closed again with care, trying to make this undetectable, and the telephone lines of persons of interest were tapped.

Nonetheless, it was understood that this surveillance is actually illegal, so it was hidden as much as possible.

Nowadays, the government institutions of most "free" states, like USA and UK have granted themselves far more rights to do surveillance of their citizens than the dictators of the past.

Detecting and prosecuting serious crime does not need any such general surveillance. The general surveillance just makes much cheaper the detecting of serious crimes, but in exchange it gives the means to the government employees to commit more serious crimes themselves.