Why was Dawn placed into an orbit that would only be stable for “decades”
In order to simultaneously obey the rules and maximize science.
Maximizing science means staying at Ceres and collecting data as absolutely long as possible. Using the last remaining hydrazine and Xenon for attitude control and propulsion to leave Ceres would have reduced the time at Ceres and data collected.
Obey the rules means keep Dawn from touching Ceres at least long enough so that if a subsequent mission were to take place, the findings would not be in any way suspect due to contamination from a previous mission. See Marc Rayman's discussion as well as @MarkAdler's comment.
Twenty years is not the time it would take for Dawn to be completely sterilized by exposure to the conditions of space.
For more on that, and a whole lot more about every step of the mission see the August 21, 2018 entry of the Dawn Journal and the rest of the entries as well, written by director and chief engineer for NASA's Dawn mission Dr. Marc Rayman (also here).
Ceres is subject to planetary protection, a set of standards designed to ensure the integrity of possible future "biological exploration" of the alien world. That terminology does not mean there is biology on Ceres but rather that that exotic world is of interest in the field of astrobiology. Ceres was once covered with an ocean and today harbors a vast inventory of water (mostly as ice but perhaps with some liquid still present underground). It also has a supply of heat (retained even now, long after radioactive elements decayed and warmed the interior), organics and a rich variety of other chemicals. With all these ingredients, Ceres could experience some of the chemistry related to the development of life. Scientists do not want to contaminate that pristine environment with Dawn's terrestrial materials.
Not all solar system bodies need such protection. The Moon, Mercury and Venus, for example, have not been of interest for searches for life or for prebiotic chemistry. For that reason, spacecraft are allowed to land or crash on those worlds because there is no expectation of subsequent biological exploration. Also exempt from such rules are tiny asteroids, including two that are being explored this year, Ryugu and Bennu. They are entirely unlike giant Ceres. They are often mistakenly thought of as being similar because of the oversimplified notion that all are asteroids. We will provide an illustration of the dramatic difference in the next Dawn Journal.
The planetary protection rules for Ceres specify that Dawn not be allowed to contact it for at least 20 years. There is a common misconception that the time is needed to allow the spacecraft to be sterilized by the radiation, vacuum and temperature extremes of spaceflight. That's not the case. Many terrestrial microbes are impressively hardy, and there is good reason to believe that some that have taken an unplanned interplanetary cruise with Dawn would remain viable after much longer than 20 years.
The requirement for 20 years is intended to allow enough time for a follow-up mission, if deemed of sufficiently high priority given the many goals NASA has for exploring the solar system. Two decades should be long enough to mount a mission that builds on Dawn's many discoveries. We would not want such a hypothetical mission to be misled by finding microorganisms or nonbiological organic chemicals that were deposited by our spacecraft. As we'll see below, the deadline for another mission to get there before Dawn contaminates Ceres is likely to be significantly more relaxed even than that.
Earlier this year, when the team was figuring out how to fly to and operate in an orbit like the one Dawn is in now, much of their work was guided by this planetary protection requirement. We did not want to enter an orbit that would not meet the 20-year lifetime. We could not take the chance of going to an orbit with a shorter lifetime and plan for subsequent maneuvers to increase the duration. We were not sufficiently confident Dawn would have enough hydrazine to remain operable long enough to make its observations and still be able to change its orbit.
The team studied elliptical orbits with different minimum altitudes. Trajectory experts investigated the long-term behavior of each orbit as Ceres' irregular gravity field tugs on the spacecraft revolution after revolution, year after year. Like Earth, Ceres has some regions of higher density and some of lower density. As Dawn orbits over these different regions, they gradually distort the orbit. The analyses also accounted for the slight pressure of sunlight, which not only can rotate the spacecraft but also can push it in its orbit. An orbit with a minimum of 22 miles (35 kilometers) was the lowest that the team was confident would comply with planetary protection, and that's why Dawn is now in just such an orbit.
And after 20 years? Calculations show that even over 50 years, the orbital perturbations are overwhelmingly likely to be too small to cause Dawn to crash. In fact, there is less than a one percent chance of the orbit being distorted enough that Dawn would hit Ceres. In other words, our analysis gives us more than 99 percent confidence that even in half a century, Dawn will still be revolving around Ceres, the largest object between Mars and Jupiter, the only dwarf planet in the inner solar system and the first dwarf planet discovered (129 years before Pluto).
Leaving the remarkable craft in orbit around the distant colossus will be a fitting and honorable conclusion to its historic journey of discovery at Vesta and Ceres. Dawn's scientific legacy is secure, having revealed myriad fascinating and exciting insights into two quite dissimilar and mysterious alien worlds. This interplanetary ambassador from Earth will be an inert celestial monument to the power of human ingenuity, creativity, and curiosity, a lasting reminder that our passion for bold adventures and our noble aspirations to know the cosmos can take us very, very far beyond the confines of our humble home.