If you're honestly worried about being bombed then you need to buy radar.

With your logic all I have to do is take the additional step of disabling your cellular infrastructure before I steam up to your port.

This is not a tactical solution. It can only be for convenience or cost savings. In that realm, AIS is the obvious answer.

Ukraine war shows improvised capability from cots hardware can have a meaningful impact. Probably easier to get 5g cell tower infrastructure than dedicated military radars.

You're telling me Ukraine can't get dedicated radar? A non weapons package that any Western nation would sell to them without reservation?

Again, probably easier to destroy 5g cell tower infrastructure than dedicated military installations.

Of course they can, though not as easily as you seem to think I suspect. Radar technology is very secret. Regardless, it’s a matter of numbers. I’m sure the Ukranians would love to field unlimited predator drones if they could. In reality, they field DJI and other commercial drones en masse because of availability. Only relatively recently have they got their own cheap mass produced drones online. These drones are also easier to destroy than a predator, so by your logic, why would they have invested in this? Wunderwaffe never wins wars, logistics do.

It can also be used for defense in depth. Each additional sensing system which must be disabled before an attack is an additional barrier.

AIS is not mandatory for all vessels, and in any case it can fail both on the vessels themselves and in the control center. Just for normal safety purposes you would want to have a secondary system to be able to continue operating busy ports.