I write R daily and have always used = for convenience. I can count on 0 hands how many times that has affected me. <<- has its uses though!
When I started, = was the norm for assignment, with == for evaluating equality. Later I started noticing that people were recommending <- based on scope concerns, but it was kind of subjective preference. Now I see articles like this saying that the "preferred use" is <-, and some people don't even know about =.
I agree = versus == can lead to tricky errors in code, but I still prefer = for various reasons in general in languages (although I get the scope arguments).
The reason is that = is shorter, and assignment is an definitional equality, which to my impression is the whole point of programming in general usually. As others have suggested here, between = and ==, = is by far the more commonly used operator in meaning, so to me it makes sense to use = for succinctness.
In math, there is an "equal by definition" operator, with three lines, so I could see that, but keyboards don't have that, so it's more steps. := is also a kind of standard "by definition" usage as discussed in the article, but again, it's more steps. I'd still prefer that over <- in R.
It really wasn’t until Google published their style guide and then later when Hadley published his that I started seeing = in heavy usage.
To your point about = being shorter I wonder if the difference was that a lot of the folks I interacted with were using Emacs with ESS which would interpolate _ to <- so they wouldn’t have noticed? Just a theory.
Either way I was taught from some of the R creators that <- was evil and to be avoided. It wasn’t until I stopped using R regularly that I switched, it became too mentally taxing to change assignment operators when I bounced between languages
x = x+1 is really confusing because of the way kids learn math. The notation x <- x+1 at least conveys the idea of taking a value and storing it somewhere.
I spent some time as a package author, that’s where I found <<- to be the most useful