"So is it your conclusion from this, that it's only the Old Testament scripture that are profitable for doctrine, for reproof, for correction, for instruction in righteousness?"
No. ;)
Also,
Matt 19:16-17
16 And, behold, one came and said unto him, Good Master, what good thing shall I do, that I may have eternal life?
17 And he said unto him, Why callest thou me good? there is none good but one, that is, God: but if thou wilt enter into life, keep the commandments.
Mary was alive when Jesus made this clear statement. Jesus Christ is God in flesh.