My observation is that although I was breastfeeding most of the time and the
baby had free access to the breast he still initiated thumbsucking at one
point early on but that whenever I saw him do that I immediately put him to
the breast and within two weeks he stopped thumbsucking and never returned.
Perhaps mothers who aren't conscious of this connection but who are bf
freely might miss it and end up with a child who sucks a finger(s).
Obviously sucking at the breast is more satisfying than sucking on fingers.

Jennifer