To use an analogy, microevolution is like putting one foot in front of the other. All you do is repeat the process and it can get you from your front step to the curb, or it can get you all the way to the shop down the block. The steps in evolution are mutations, and it is the accumulation of single mutations that accounts for the evolutionary distance travelled. Macroevolution is the accumulation of microevolution.
Speaking for myself, there is absolutely no reason why we would expect a tree of life if design is true. Why would a designer not use certain designs where they make sense? For example, there are probably situations where a vertebrate species would benefit from having an eye with a forward facing retina like that found in the octopus. So why not do it? Why would I, as a designer, be forced to use an eye with a backwards facing retina in all species that have a backbone? Why wouldn’t I mix and match different parts in a way that didn’t form a tree of life? A tree of life makes no sense from a design perspective.
In one hand we have is a natural and observable process that can only produce one pattern, a tree of life. In the other hand we have a proposed process that can produce millions of different patterns that aren’t a tree of life. What do we observe? A tree of life.