That's exactly it. Because I think I'm alone doesn't mean I am.

My blind spot indicator in my car has 100% prevented me from merging when I shouldn't, though I can't say with certainty how often. I'm a little embarrassed to say, too, that I've been honked at in the past for merging when I shouldn't. I think twice in 20 years, but still. I have no reason to think those incidents couldn't have been severe.

No one is 100% sure what's surrounding them at any time, plain and simple.

This is one argument I see for autonomous driving which actually seems valid to me. Those vehicles will be able to see far more than a human facing forward with a few mirrors. At the moment I think the problem is that the human can make far more sense from what they see, though.

Don't rely on your blind spot indicator, it's there to help, not replace a blind spot check. They often don't pick up motorcycles and especially not bikes. I have seen them not light up many many times when I'm passing through someone's blind spot on my motorcycle, when the car in front of me does set it off...

I definitely don't rely on it, I agree with you. I think what happens is I have moments of thinking I should be able to merge, then checking the indicator and realizing I was wrong. In those cases it's very likely I'd do a thorough check, but it's a great reminder otherwise that my gut sense of my surrounding is incorrect, and I do need to check thoroughly and, for lack of a better explanation, assume I'm always wrong until I've fully checked my surroundings.

If the sideview mirrors are set properly, there shouldn't be a blindspot.

If you can see the side of your own vehicle in your side view mirrors, they are set improperly.

When set properly, cars in the lane next you on either side should transition from your rearview mirror to your sideview mirror to your driver or passenger window without any gap in coverage.

The sideview mirrors are to see other cars, not your own car. You're in your own car, so there is no reason to set it to see the side of your own car. If you can see the side of your own car, then angle the mirrors out until you can't. Next time someone passes, check to make sure you see them transition your mirrors properly, it they don't tweak your mirrors a little more.

You're not wrong, but as a motorcyclist, I'd prefer everyone look back regardless of how correctly their mirrors are adjusted.