Some of the newer missiles are designed to avoid jamming techniques by firing them into an intercept vector without active radar control and then allowing the seeker to acquire the target when it is too close for jamming to be effective.
It seems logical that such a missile might find it more difficult to acquire a smaller more agile target like an SU-25 and instead acquire a larger, more stable target like a passenger plane.
“It seems logical that such a missile might find it more difficult to acquire a smaller more agile target like an SU-25 and instead acquire a larger, more stable target like a passenger plane.”
You might think that, but there is an important assumption involved. The seeker is given a finite set of parameters that the target must match for the seeker to initiate an interception. Exactly what these are depend upon the missile system and neither of us have this type of information necessary to do a complete analysis of whether a specific engagement scenario is plausible or not.
For the engagement to be plausible, you must assume that both aircraft match those parameters. At the very least, both aircraft would have to be very close to each other, flying in the same direction, and at similar velocities for a match to occur. The definition of ‘very close’, ‘same direction’, and ‘similar velocities’ would vary depending upon the system. Systems designers take great care that these terms are not so loosely defined that it is highly unlikely that a missile would engage any other target except the one it is fired at. You would rather miss than engage the wrong target. The SU-25 would almost have had to be flying in formation with the airliner for this scenario to work.
Missiles are very expensive. You take great pains to ensure it engages the target you want it too. You do not let it wander off because it might shoot down one of your planes.