Also, Jackson did NOT have firm control. He lost the ball completely but was able to re-grab it.
But even if he had firm control, so what? If his foot came down OOB there’s no catch. If he got shoved so that his foot came down OOB there’s no catch. If he jumps, gets firm control, and a defender forces him OOB before he lands there’s no catch. There’s nothing special about “firm control”. You still have to complete the catch. And that means avoiding OOB until you’re done.
And many sports have “OOB is conductive” rules where a player touching OOB is considered to be entirely OOB as is anything the player touches. This is hardly unique to football.
Finally, you worry too much. No defender is going to regularly position himself OOB to try to make a play. For one thing, it’s illegal to move down the field OOB. For another, they’re going for the receiver, who is in-bounds. Positioning yourself on the sideline to try to pull off something like this will burn the defender far more than it will help him.
One thing that does surprise me about sideline plays, however, is that since the elimination of the forceout rule DBs who are near a jumping receiver at the sideline don’t simply concede the ball but then just shove the receiver to land OOB. Seems a higher percentage play than challenging the ball.