A galaxy is a whole bunch - gazillions - of stars and stuff in orbit around a common centre. It reached this equilibrium point through millions (often billions) of years of things either going into stable orbits, or being thrown out of the galaxy (for going beyond escape speed) or falling into the centre (for not going fast enough).
What you see is the result of this stabilization.
IF (this is a big if) something should happen to break the equilibrium, it would take millions of years for "whatever" to happen.
In our case, our Galaxy will interact with the Andromeda galaxy in (roughly) three billion years. Although we call that a collision, there will be very few actual star-on-star collisions (there is simply that much space between each stars).
However, due to tidal effects of one galaxy on the other, some streams of stars will be (very slowly) thrown out, while others will fall into the black holes at the centre of each galaxy. Most of the stars will simply enter into chaotic orbits (they will stabilize over millions of years) to form an elliptical galaxy.
Of course, whatever happens to a star will entrain its planetary system (the planets simply go along for the ride).
This is so slow that an average human being living an average lifetime will not notice any difference.
Look up "antennae galaxies".