There's a second motivation in my opinion. Code might work today without the change, but it could be because the author originally wrote buggy code, caught it in testing, and had to waste time tracking it down and understanding nuances that don't need to be there. Once they figured that out, they implemented an ugly workaround (adding an extra function parameter to a goroutine or shadowing the loop variable with n := n).

Good language designers want to avoid both wasting developer's time and requiring ugly workarounds. Making a change that does both, especially if it doesn't break old code, is great imo.