Don't worry about all the "what if's."
I taught both my kids sign language from the time they were 6 months old. Both my kids started "speaking" at their OWN pace. Each child is different. NO child has the same pacing nor timeline for "talking."
Sign language is NOT a "crutch." In fact, it is a help and just ONE way of communication. A child, will NATURALLY "stop" signing when they are speaking more. If your child is not yet "talking" as you feel he should, then ask your Pediatrician.
There is a difference between talking and the child understanding what is spoken TO them. Talking... does NOT INDICATE "smartness" or "ability" of a child. AND it is often gender based as well. ie: boys talk later than girls. Girls mature quicker and faster than boys, generally. MANY boys begin to talk later.
I have a boy and a girl. My daughter was talking before 1 years old. My son, was later... it was not until about 3 years old that he took off in talking. He is now MORE talkative than his sister!
Einstein... did not talk until 3 years old. I believe Mozart was the same. And, they are GENIUSES! Nothing was "wrong" with them, except that they were so genius that others did not understand them as a child.
My son... did have a Speech Therapist. He had a TOTAL overall assessment done... and he was even found to be ADVANCED in most areas, but his "speaking" was delayed. Not "delayed" in a bad way, the Therapist said he was just "shy" about talking and not ready yet. There were NO DEVELOPMENTAL "problems" that he had whatsoever. But so in time, and with encouragement... he ON HIS OWN, began talking as his confidence grew. ALSO, though, because he is advanced in several areas for his age... his brain was just concentrating on other areas.... and talking to him, was just not ready or as important. He was never frustrated though... and in fact, he DID communicate very well, with signing and gesturing and many "functional" words that was useful for HIM. It was amazing, seeing him very consciously use the FUNCTIONAL words that he wanted to... to tell us what HE needed. And, he could always express his feelings. So... the moral is: even though he "talked" later.... he was very able to consciously pick and choose what words HE needed to, to navigate himself and he understood everything told to him very well. Once he did start "talking".... we realized that he was just storing EVERYTHING in his brain... and he blew us away with his vocabulary and communicative and expressive language ability. As I said, he is now a VERY talkative AND expressive 3 year old. Much more so than other kids his age. AND, my son, both my kids, are bi-lngual too.
No, sign language does NOT hinder "talking" nor is it a "Crutch." In fact, the Speech Therapist my son had... encourages signing. And, even "babbling" is a precursor to talking and even "singing" is "talking" too. ALL OF THESE THINGS, go hand in hand and work together... towards "talking" as adult expectations are, or what "we" expect our child to say.
A child, will talk when they are ready to.
But if you feel your child has a developmental problem... then you talk to the Pediatrician.
There is a certain progression in "talking" for a child... certain alphabets/sounds are harder to pronounce than others, some word combinations require a lot of tongue/mouth coordination and some letters have "front of the mouth" or "back of the mouth or throat" sounds... which at certain ages a child will naturally progress to. The word "milk" for example... is not easy to say or pronounce. So don't expect him to say it.
ALSO, kids this age OFTEN do not articulate nor pronounce words perfectly... yet. It is all about mouth/tongue/throat coordination too, in conjunction with making the "sound" of the words. MANY elements are involved.
All the best,
Susan