The idea of post-humanism doesn't hinge on becoming purely software, it simply means becoming partly or entirely independent of biology. An AI or mind upload in a non-networked robotic body (android form or otherwise) would have few or none of the security problems you mentioned, and would still obviate all of the business of terraforming, etc. Synthetic neurons are already working in labs today.

But beyond this specific objection, I think the big gap is failure to grok the real implications of exponential trends. Now sure, you can argue (as folks have been doing since the 80s when I first got into futurism) that Moore's Law is about to end and price-performance of computation is about to hit a wall. But if that doesn't happen, and the 115-year exponential trend continues for another 50 years, then computers more powerful than your iPhone 6 will be the size of blood cells by 2065. And because of machine labor, their marginal cost will be nearly zero.

The implications of that trend are enormous, not least for transhumanism. Machines of that complexity and size will allow us to interact with and modify biological function in a way that is difficult ill to distinguish from magic today. In the same way that a 2010 smartphone with all of its functionality for $300 would have seemed completely impossible within 30 years to almost any observer in 1980, even engineers, the idea that we will be well into cybernetic enhancement and brain-machine interfaces by 2045 seems impossible today to many folks - even engineers. And this is the case even if we ignore any possibility of a AI singurity-style intelligence explosion. If we include AGI, then if we aren't all quickly killed then the transition to post-human world is going to happen even faster.

It is easy for folks to forget what is actually possible when a huge amount of resources are intelligently directed towards a goal. The Apollo program lasted only a decade. Three gorges dam was built in about the same time. All of WWII took place in six years. And that's just with human labor. So while I'm sympathetic to concerns that we won't have a nuclear reactor for a Mars mission until 2032 because there are only a few hundred people with a few billon dollars working on it, I think it is very easy to underestimate what machine labor with human or greater intelligence could accomplish in an astonishingly short period of time. Machine labor (both macroscopic and microscopic) with access to atomically-precise manufacturing technology could terraform Mars in a matter of days or weeks, for example, if they were allowed to self-replicate and had a sufficient source of energy such as nuclear fission or fusion.