The DC value of an oscillating system can be thought of as stable, but whether the dynamic behavior is stable or unstable comes down to the damping factor. Reasonable people can differ, but there certainly seems to be a tendency to underdamping recently.

> Is this not stability?

No? Maybe one could claim it results in some sort of equilibrium, but radically changing policy on important topics such as environmental protection, corporation taxes, infrastructure, etc. every 4 years is not stability. Not to mention that every topic becomes an instant partisan issue - if one party says something is good, the other has to fight against it, regardless of merits.