Labyrinth of Target Architecture

I was thinking and working a lot about target architectures. Usually it is expected that you as architect will define a kind of map, which can be interpreted anyone in the company and reach the shiny targets on low cost zero time and no business outages. I would not say that it is not possible, rather if you plan cost, timing and migration phases but it is far to be simple. The real issue is that a target architecture is a collection of demands driving to ideal situation, which is not necessarily the place you would be; and the price tag, the duration and the full freeze of "disturbing" changes to reach the ideal situation makes most of the target architectures unreal. Let's see why!