...“สลาย” มีความหมายว่า “หมดไป”...

Perhaps, we should make clear the difference between

(a) 'breaking down into very small pieces' but every piece is still plastic (like iron), and

(b) 'degrading into smaller particles of different properties (like iron oxides), and some may be

(c) 'becoming raw materials (for re-use) in natural cycles

Obviously, it would be good for plastic bags to be of (c) type=หมดไป rather than (a) type=ไม่หมดไป. ;-)