Ulevo
If you lose nothing for at least attempting it, what harm have you done to yourself? None. If anything you get some percent which is better than no percent.
No. What you lose has to do with opportunity cost.
In that same situation, against a player who knows how to DI in this match up, that jab lock will not happen. By going for the back air, you will net 7% and your opponent gets out for free. Meanwhile if you down throw to up smash, he cannot DI or get out, you gain positional advantage, and it is much harder for him to DI the impending dash attack that will start the death combo. Even if you landed the back air, your opponent just needs to hold away and it will mess up the dash attack 1 start to the combo. You could use dash attack 3 or up tilt 2, but those also have different % ranges because of height and angle variances.
Like, this is very simple. 1: It does not combo. 2: If they DI away and jump it fails. 3: If they DI away and air dodge it fails. 4: If they DI away and aerial it will likely trade or fail. 5: If they tech, it fails. 6: If they mash A, they will tech bounc an aerial, you take damage, and it fails. 7: If you land the back air and jab lock them, if they DI your follow up and you use the incorrect one, it fails.
Good mix ups entail 50/50's. If Sheik down throws you, you are either taking damage and potentially dying for choosing option A, or dying for choosing option B. At no point should this be working against competent players who have Meta Knight experience.