I think the system was designed intentionally so, that the player must be there. If the cell is not loaded because the player is far away, there can be no fight at all, and they did not implement a 'auto resolve feature' where defenses and weaponry of the settlers are compared to the attackers strength. This would be a purely scripted evaluation, not a real fight. It is possible to do this, but they seem to have decided against it, because they wanted the 'action', they wanted the player to participate.
I think, that the defenses and well armed settlers are only here to determine the frequency of attacks and to help you, when you arrive to defend the settlement. If you have a lot of defense positions, turrets and well armed settler it will be easy. This way the settlers become your temporary companions while fighting off the attack. It is a design decision.
If it were different, you often would only hear 'there is an attack', and 'settlers could fight it off', and many players would not find that very interesting. A solution in between would be fine.
I think, that the system should take into account, that you may be unable to respond. If you in a quest in an inside cell, you cannot easily fast travel. Since you can be attacked while in dialog, it may also be possible for a settlement attack while you are in dialog. They do not stop the engine while in dialog!
Spoiler In Cambridge I was talking to the guy who sells dogs, and while talking to him (I had saved just before that, because I waited for this random event) a Supermutant Suicider came running around the corner. Since I had a save before talking to the NPC I did not care and waited what would happen. The Suicider ran towards us and triggered his bomb. He attacked the dog. The NPC ended the dialog at this point, but we both were only lightly injured and the dog was 'dead'. But not really dead, since he got up after a few seconds and I could continue the dialog and buy the dog.
But this behavior may be a problem with settlement attacks.
I think that settlement attacks are simply radiant quests and work like a radiant quest: The quest is started, given to the player and the setup is done. The timeout is set to a very short time. Regrettably the quest start is not visible like other radiant quests, where you have sound and a big label popping up. You only get one small message, and it seems that this message often is suppressed by something you do, or that the quest does not start at all, since in many cases you not only miss the quest start, and do not see it under Misc Quests listing in the PipBoy, there is also no fail message. But the settlement is damaged. Maybe this cases are wandering monsters, and not the radiant quest, but the implemented mechanism how Monster versus Settlement is resolved (by damaging the settlement) still is active.
So - I think it is best not to care too much. I build defenses, but not perfect, it is too much work anyway with this awful interface. I put turrets, and set defense posts and settlers as guards. Guards are helpful if you go there to defend: They cannot die, and as long a they are standing they fire on the enemies and this way take pressure from you. And if I am somewhere doing an important quest I ignore the attack, since in the end in the worst case the settlement has many damaged things, but all the settlers survive and you can build up the settlement again. Some things - as reported in this thread and elsewhere here - seem to repair themselves. (Maybe this is some automatic repair from the settlers? Would be interesting to see if it consumes resources)
I think, that the entire system is hard on the limit what this engine can do. I am happy with the settlement attacks, and will respond in most cases, if I have time enough. I want to be able to end dialogs, put my character in a stable environment and prepare before I travel to the settlement. But there should be a more thoughtful resolve, if I do not respond, and the settlement attacks should happen not too often. If I have all the settlements (30?), it is not fun if I cannot do something else for an hour without an attack.
As always with Beth games, they tried to push the limit, and as always they did not think things through fully. I think this is also intentional - they want to see how things are received and how the work 'out in the wild' with many players. And then they will try to resolve things, which many people do not like or have trouble with. So I hope they will react to the many settlement issues, or the settlement concept will be stillborn. Soon there will be mods which disable attacks at all (one seems to exist already), but I would not want that. I think the concept is fine, but it needs some finishing touches, not only with the attacks but with many other things too.
What also needs work is the 'job and schedule animation' from settlers. Often the animation do not start. Also the path finding from settlers seem in sort of a 'relaxed mode'. Often I see settlers trying to walk from one guard post to the next, but hanging on a corner of the guard post they are trying to leave. The keep walking in place, with does look awful. But there are path finding algorithms which can resolve such situations easily, or no companion would be able to follow you. I think, that the engine tries to conserve engine cycles, because on weaker machines (or consoles) a full settlement may be too much to manage. I had a guard post built so, that I could not walk into it with out jumping. As a result the settler assigned to it kept standing in the middle of the settlement and doing nothing. As soon as I found this and corrected it, the settler started to walk to his post. NPC schedules seem to be done so, that if something hinders it the schedule is simply suspended. And if you arrive via fast travel in a settlement, the settlers will all just start their schedule. If you walk towards a settlement (i.e. the cell is loaded before you can see it) then the schedules are running and it looks better.
All in all the settlement system is a true innovation in this kind of game and I hope Beth sees enough interest to develop it more fully!