I do also find that only using a turn signal when others are around is a good reinforcement to always be aware of my surroundings. I feel like a jerk when I don't use one and realize there was someone in the area, just as I feel like a jerk when I realize I didn't turn off my brights for an approaching car at night. In both cases, feeling like a jerk reminds me to pay more attention while driving.
Signalling your turns is zero cost, there is no reason to optimize this.
In my experience, I'm best served by trying to reinforce awareness rather than relying on it. If I got into the habit of always using blinkers regardless of my surroundings I would end up paying less attention while driving.
I rode motorcycles for years and got very much into the habit of assuming that no one on the road actually knows I'm there, whether I'm on an old parallel twin or driving a 20' long truck. I need that for us while driving and using blinkers or my brights as motivation for paying attention works to keep me focused on the road.
Signaling my turns is zero cost with regards to that action. At least for me, signaling as a matter of habit comes at the cost of focus.
I have also ridden motorcycles for many years, and I am very familiar with the assumption that nobody on the road knows I exist. I still signal, all the time, every time, because it is a habit which requires no thinking. It would distract me more if I had to decide whether signalling was necessary in each case.
What do you mean by "comes at the cost of focus", there? Do you mean you are more distracted by having to use your indicators?
Maybe you're just not a very good driver, if you're so distracted by the basic controls of the vehicle.
Seriously: signal your turns and stop defending the indefensible, this is just silly.
That is a very bad habit and you should change it.
You are not only signalling to other cars. You are also signalling to other road users: motorbikes, bicycles, pedestrians.
Your signal is more important to the other road users you are less likely to see.
Always ALWAYS indicate. Even if it's 3AM on an empty road 200 miles from the nearest human that you know of. Do it anyway. You are not doing it to other cars. You are doing it to the world in general.
Here is a hypothetical: A loved one is being hauled away in an ambulance and it is a bad scenario. And you're going to follow them. Your mind is busy with the stress, trying to keep things cool while under pressure. What hospital are they going to, again? Do you have a list of prescriptions? Are they going to make it to the hospital? You're under a mental load, here.
The last thing you need is to ask "did I use my turn signal" as you merge lanes. If you do it automatically, without exception, chances are good your mental muscle memory will kick in and just do it.
But if it isn't a learned innate behavior, you may forget to while driving and cause an accident. Simply because the habit isn't there.
It's similar for talking to bots, as well. How you treat an object, a thing seen as lesser, could become how a person treats people they view as lesser, such as wait staff, for example. If I am unerring polite to a machine with no feelings, I'm more likely to be just as polite to people in customer service jobs. Because it is innate:
Watch your thoughts, they become words; Watch your words, they become actions.
This has a failure state of "when there's a nearby car [or, more realistically, cyclist / pedestrian] of which I am not aware". Knowing myself to be fallible, I always use my turn signals.
I do take your point about turn signals being a reminder to be aware. That's good, but could also work while, you know, still using them, just in case.
I've been driving for decades now and have plenty of examples of when I was and wasn't paying close enough attention behind the wheel. I was raising this only as an interesting different take or lesson in my own experience, not to look for approval or disagreement.
Just consider that you will make mistakes. If you make a mistake and signal people will have significantly more time to react to it.