In Young's double-slit experiment, the interference pattern arises from the superposition of light waves coming from two coherent sources (the two slits). When the waves from the two slits meet, they can either interfere constructively (resulting in a bright fringe) or destructively (resulting in a dark fringe) depending on their relative phase.
The amplitude of a light wave may indeed decrease with distance from the point source due to factors such as spreading out of the wavefront as it propagates. However, in the context of Young's double-slit experiment, the decrease in amplitude with distance is generally not significant over the short distances involved in the experiment.
Destructive interference occurs at certain points in the interference pattern when the waves from the two slits have a phase difference of half a wavelength (λ/2) or an odd multiple of half a wavelength (3λ/2, 5λ/2, and so on). In these cases, the waves are said to be "out of phase" and can cancel each other out, resulting in a dark fringe.
The decrease in amplitude of the light wave with distance does not prevent destructive interference from occurring because the phase difference is the critical factor for interference. As long as the waves from the two slits maintain the appropriate phase relationship, they can still undergo destructive interference even if their amplitudes decrease.
It's worth noting that factors such as diffraction and interference effects can become more complex when dealing with specific scenarios and longer propagation distances, but for the basic understanding of Young's double-slit experiment, the decrease in amplitude over short distances does not hinder destructive interference.