That is only one possible definition of the word "heir" and not the one used in the scriptures.
The Merriam Webster dictionary's second definition of the word "heir" is,
"one who inherits or is entitled to succeed to a hereditary rank, title, or office"
And the example it gives is "
heir to the throne."
Definition of HEIR
You would think that someone described as the "heir to the throne" would not receive only the literally throne, but the title and authority of the one who sits upon the throne.
The parable of the prodigal son shared by the Lord Jesus Christ claims that the faithful son would receive all that the father had.
"And he said unto him, Son, thou art ever with me, and all that I have is thine." (Luke 15:31)
If this father happened to be the Supreme King and Creator of all the Universe, what would the Son receive?
Keep all of this in mind when we bring it back to Romans 8,
"For as many as are led by the Spirit of God, they are the sons of God.
For ye have not received the spirit of bondage again to fear; but ye have received the Spirit of adoption, whereby we cry, Abba, Father.
The Spirit itself beareth witness with our spirit, that we are the children of God;
And if children, then heirs; heirs of God, and joint-heirs with Christ; if so be that we suffer with
him, that we may be also glorified together." (Romans 8:14-17)
Paul decreed that those who are led by the Spirit of God are the sons of God and therefore
"heirs of God", not "inheritor of something had by God", but "heirs of God".
And we all know that the word "God" is a title bestowed upon the Supreme Being, so if we follow the Spirit and suffer with Christ we can inherit the title and authority of God.
No one said anything about "automatic". There is obviously a process described here about how we are to become like God.