Can your eyesight get bad again after laser eye surgery like SMILE?
Not sure if your question is asking about SMILE in particular or just about laser vision correction in general. Fortunately, the answer is basically the same for LASIK, PRK, and SMILE. These procedures do not actually "make bad vision good" although that is indirectly what does indeed happen. They work by re-shaping the cornea, the clear, lustrous "dome" overlying the pupil and iris. Since the cornea is responsible for about 2/3 of your eye's focusing ability (nearsightedness and astigmatism) are usually quite able to be adjusted down to near zero from this corneal adjustment.
But eyesight is not purely a matter of the cornea alone. There is also a natural lens just behind the pupil that can change later in life after laser vision correction. And there are non-focusing problems in the eye that can also occur which will decrease one's vision later in life, e.g. macular degeneration. Most of the time any decrease in eyesight later in life will be caused by one of these, NOT by the laser vision correction "wearing off."