The US are currently concentrating air forces around Syria in probable preparation of the conflict going hot in the air after the Russians said they'll treat US planes as hostile.
The Russians however don't actually need to scramble fighters against the Americans. As far as I know they have some S400 AA missile complexes in Syria ATM, which would be more than enough to take down some US plane and create a huge diplomatic crisis.
The downed aircraft was an SU-22 fighter bomber. While a relatively good aircraft and pretty fitting for taking out rebels and terrorists in ground attack missions, I doubt it can be a real threat to an F-18 since they claimed that the plane was shot in self defense?!
You mean that Americans would not shoot down FOREIGN military jets, that are in United States air space without UN approval?
Please, tell me, what should Syrians do exactly? Sit idly and die under guided missiles for their own good?
I have problem with the fact, that you say Syrians "would create crisis" by the fact, that they defend
their own country. United States are in this case the agressor. Internatinal law clearly bans any such actions on soil of foreign countries.