> Your argument kind of falls on itself there --> We can surveil them in person, but not digitally? Why?
Because, before the internet, when surveillance was a lot of work, this prevented the abuse that is mass surveillance.
Only now do we see how much democracy relied on this natural limitation of state power for civil rights. It was never just the need for warrants that maintained civil rights. Remember the Verizon FISA court order authorized surveillance of millions, and that was just one order of hundreds.
Power corrupts. State power should be sufficient, but minimal. Being allowed to do physical surveillance only is sufficient to reduce crime rates to the point where most people can safely neglect that crime exists at all, and it is less power because it does not scale.
You mention proportionality yourself. Proportionality means that something is not done if the same objective (finding a given murderer) can be achieved in a less rights-infringing way. Proportionality at the policy level, means that a surveillance power may not exist, if its objective (such as safety from murder, i.e. low rates of murder, high chance of finding murderers etc.) can be achieved without the power or with a power that is less likely to be abused or that infringes rights of suspects less. (I admit this is somewhat of an editorialization; the technical meaning of proportionality is in [1]. To be clear the existing legal proportionality principle does not try to directly minimize power; but it does usually present an obstacle whenever new powers are created by law)
Limiting power is simpler and less error-prone than allowing power and adding control structures like warrant requirements for the power. It is thus better, if the outcome is the same.
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[1] https://en.wikipedia.org/wiki/Proportionality_(law)#European...