The principle of prokaryotic plenitude

From Edgar Andrews (Who Made God?):

Numerically speaking, the vast majority of living things on planet earth are simple organisms like bacteria. These creatures are so well adapted to a huge variety of environments that they exhibit neither the need nor the tendency to evolve into earthworms or earwigs - nor yet to vanish into evolutionary obscurity as they are superseded by later models. If, as is alleged, evolution is driven solely by reproductive efficiency and optimized survival, then by rights all life forms should be striving to become bacteria, which perform superbly by these criteria. Yet in spite of having near perfect adaptation as a class of living creatures, evolutionary theory insists that bacteria (or something very like them) did evolve - otherwise you and I would not be here. Laboriously, they hoisted themselves up the evolutionary tree, becoming all the while decidedly more complex and environmentally 'picky'. That is, they evolved into vulnerable creatures that, according to the fossil record, regularly got themselves snuffed out by extinction. Why, I wonder, would they do that, seeing that their un-evolved bacterial cousins lived on in happy adaptation to every conceivable environment, sublimely unaware of the hazards of climbing trees?