We, the Zetas, are sometimes referred to as one of the oldest alien races visiting Earth, but in terms of age compared to most, we are in our youth. The Zeta form, and culture, has been around 8.5 times longer than man, here on earth, which makes Earthlings mere toddlers. We are older than the Pleiadeans, in their present form, but the Nordics predate us.
What determines how long a race survives, essentially unchanged in form and retaining the same culture? Many things, but of primary importance is how well the genetically engineered form is adapted to its environment. A poor adaptation almost begs for change, and change comes, either by the sickening and extinction of the race or by intervention in the form of supplemental genetic engineering. Of secondary importance is the complexity of the culture. With cultures, simple is not best. Like complex genetics that allow an organism many possible responses to a situation, cultures require depth to survive. A soldier given only two alternative actions is likely to end up dead, when sent into battle, but one allowed to rely on his own resources will survive. We are speaking here of freedoms, so that the culture can adapt over time, moving in new directions. Rigid cultures do not survive.
Has the Zeta culture changed over time? Most certainly. We were once more war prone, rising to the slightest challenge and letting nothing slide. In this regard, this is the one characteristic of the giant hominoids who inhabit the 12th Planet that is most like our ancient selves. The remnants of this culture can be seen within us, as even today we do not, as you say, roll over easy. We defend ourselves, some would say vigorously. Our culture has moved over its life, steadfastly in the direction of supporting scientific research, as this is a pastime we all enjoy immensely. Some cultures, as the Pleiadeans, have always placed a strong value on social functions, giving them priority over almost all activities. As this is something they enjoy immensely this has, if anything, grown in importance for them as time went on. Consequently, the Pleiadeans frequently act as greeters for new galactic citizens, and are here on Earth acting in this capacity now.