The way I see it, there is no tree, no hierarchy, but a huge, flat plain, that has ripples, contortions, and convolutions, but all the life is life.
Slime mold is easy to see and moves pretty fast so we can watch it in action. There is plenty of stuff moving around underground where we can't see it. It could be that a system that uses fluid dynamics could last a lot longer than a system based on electricity. The bacteria go back 4 billion years, the date of first appearance of fungi keeps getting pushed back, around a billion years now. That is probably not too accurate as the oldest sponges are now coming in at around 890 million years.
Going with the idea that nothing is as it seems to be. Slime mold and other fungi could be multi cellular creatures in the fluid dynamic world. Their cells are able to restructure themselves to perform different functions for the benefit of the entire group. Slime mold can even fling pieces of itself across crevices that block it's movement in that direction. That is a whole lot of coordinated action, not to mention the fact that it flings the pieces of itself across the crevice and not just anywhere.
One reason we may not be able to understand the dynamic fluid life could be along the lines of the differences between sound waves and light waves. They both follow similar rules but in different ways. Some of the cells in our bodies communicate with chemical messages sent out cell by cell, which travels relatively slow like sound. Other cells use electrical impulses to send signals which originate in chemical reactions but travel faster. Those chemical reactions are molecules moving between molecules, kind of like sound, so maybe we aren't so far into the zone of electrical powered life as we think we are.