The transformation is a pleasing aesthetic, also.
On the argument that Terrans can't have advanced so quickly in the given amount of time, keep in mind that humans are advancing far faster than one would think, and I believe that that accelerated progress will only increase with the rise in understanding and technology. Long story short, it may be that the farther along you are, the faster you move on the scale of technological progress. I mean, it's not even that big of a change to the siege tank. Just some polished metal, and improved treads and cannons.