I would disagree. I would say, yes, learn to get the assessment right. You need to make sure you have looked all around for all other road users and identified if there is any reason why it would not be safe to make the turn. But that's not so much a prerequisite for indicating: It's pre-requisite for actually making the manoevre. In other words, as you're coming up to the point where you would indicate, you assess all round you to make sure it's OK in principle for you to make the turn - as long as, after making that check, you still intend to turn, you indicate anyway as a way of making your intentions absolutely clear to everyone. You don't say 'Oh, no-one will be inconvenienced if I don't indicate, so I won't bother this time.' Just indicate. It's simpler and safer [*]. Then, just prior to actually making the turn, you re-check and make absolutely sure you have identified any other road user who would be affected by your manoeuvre. And obviously, at that point, you stop and wait if any other road user has a postion/velocity that would make the manoeuvre temporarily inadvisable.
The problem with saying, 'don't always indicate because you should be training yourself to see all road users' is that you are basically deciding not to activate a safety feature that might (if you missed something when you looked round) save someone's life - on the basis that you want to train yourself better. Offhand I can't think of any other safety-critical environment where that logic would be considered acceptable. The widely accepted logic of making things safe is that you build multiple safety systems in, so that if any one individual safety component fails, the next one will kick in instead. That makes an overall failure very unlikely because you need multiple systems to fail at once.
In the case of making a turn, you have two safety systems: Your own manual checking that it is safe to make the turn, and then your indicator lights that serve to tell everyone else what you are intending to do. Both of those are important and should be used. You don't decide not to bother with one of the safety systems because the other has given an all-clear.
[*] The one important exception to be aware of is cyclists, because for a cyclist, indicating requires taking a hand off the handlebars, which slightly reduces control of the bike and can prevent braking. Cyclists therefore do have a good reason for not indicating unnecessarily.
Except the moment you do it all the time you compromise this and will progressively stop doing it. Ultimately you get to the lazy habit of signal, manoeuvre (mirror?) which is how so many seem to drive - thinking that if they signal they have a right to do it which must be accommodated.
I think you’ve over emphasised this as a “safety system”. I work in aerospace and the onion logic is well known to me. This is an indicator not some collectively thought through system.
With “if in doubt do it” and consistent rigorous practice I don’t see an issue.
On a roundabout I watch the vehicle positioning and speed far more than the indicator - most people seem to leave them on, or as I find they click off again with a bit morn turning or a lane change - so taken with a massive pinch of salt even with the best will in the world.
Based on what you've said in this thread so far, I somehow doubt you'd be driving around in bumper to bumper traffic with your high beams on and so wouldn't get that treatment!
Of course, all courses, highway code and law say that you must dazzle or cause discomfort to other road users - ie don't have your high beams on when you're right behind another car! What's your suggestion for trying to alert drivers behind that they need to check what they're doing (in the same way that anyone driving towards me with full beams on will get a cursory flash to remind them to dip their lights)
I do tend to sit back, however in congestion on multi-lane traffic that can encourage people to pull in front which then frustrates people behind and causes issues of it’s own in suddenly leaving me no space (and not wanting to precipitate an instant braking wave) so I tend to close up a bit to discourage that.
As above I was taught to look far ahead, on a typical fastish motorway line of traffic say 10-15 cars. That way I can start my braking thinking distance long before the car in front’s lights come on. It also allows me to just ease off the acceleretor if I can see it is a minor adjustment rather than a major braking and thus try and even out the braking wave or even stopping it. By flexing the gap (but not to the extremes that people who dont seem to realise the motorway has opened up again) then it feels like I’m helping keep a higher average whilst being safe.
Generally a decent gap is good as if you are looking to overtake you get a better view down the sides, over and under from being further back. You can then use that distance to accelerate up behind and as the start of the overtaking opportunity (eg car coming the other way passes) you can “pop” and fly by already at overtaking speed (I look to change up into a cruising gear as I pull out) If you change your mind you can stand on the brakes and stay behind. I overtake a fair bit in some areas I drive and I’m always amazed at how little of the distance I have allocated for myself that I actually need. Again all as I was taught. The one flipside is people seeing you close fast in the mirror and possibly panicking- but they should get the idea you will imminently go past.
One thing that absolutely breaks me is seeing people, typically on the inside lane of a motorway, slowing/braking to let those on slip lanes get in ahead of them.
I was taught that you should continue as your same speed as it is incumbent on the person joining to do so safely - and the one thing they need, in the mirror/over the shoulder looks they get, is for you to be consistent. The very last thing they need to make their decisions is you changing stuff when it is hard for them to see that and it takes precious time to identify your speed in the first place let alone again.
Hence you see these (well intentioned) see-saw things where both sides come to a halt and then get frustrated with each other whilst also causing chaos behind them.
Only if you can easily help by say pulling out, or there is no option, should you do anything.
I’ve always applied a bit of sailing to my driving - that is when you do something make it clear you are doing that by the movement you make, eg. on a boat, to show you have seen a potentially conflicting one you turn more than you need to to give a noticeably different aspect. Military drill can be seen the same way - there are no subtle moves but all clearly defined.