Passive components include resistors, capacitors and resonators. They are called passive because they do not increase signal power. In contrast, active devices do increase signal power, and include amplifiers, digital logic and oscillators.
Passives used to dominate circuit design. A tube-based TV from 1965 might have had ten active tubes and a hundred passive resistors and capacitors. Now my smart phone has millions of active transistors but only a few hundred passives. (I’m counting the discrete passives here.) The ratio of active to passive components has increased a million fold.
Why does this matter in precision timing? Timing is divided into two parts: precision oscillators for high performance applications, and resonators for low cost applications. And MEMS oscillators have been replacing quartz oscillators in high performance applications, but not quartz crystals in low cost applications. So that is active replacing active.
But that is not the whole story. Silicon is moving down in cost faster than the quartz. MEMS oscillators will soon be lower cost than quartz crystals. And actually, we are seeing that some folks are already using MEMS oscillators where they had been using quartz crystals. Someday, and that is not tomorrow or next year but is soon enough, active MEMS oscillators will replace both active quartz oscillators and passive quartz crystals. MEMS oscillators will be used in both high performance and low cost applications.
And that is another example of how it looks when silicon replaces incumbent technologies. Actives replace passives.