0.999...=1 is true in the mathematical sense, period.
However as a representation of physical world, there is a caveat. What we understand is physical world appears and behaves discretely, because at planck scale (approx. 10^-35) the distances seem to behave discretely.
Although common people don't know/ understand planck scale, they do grasp this concept intuitively. What they are really saying is that in physical world there's some small interval (more precisely, about[1 - 10^-35, 1]) which can't be subdivided further, based on our current knowledge.
Same thing applies to planck time (approx. 5 * 10^-43) too.
So people are arguing two different things - the pure maths concept, or the real world interpretation.