Excerpt taken from:
L’Osservatore Romano
Weekly Edition in English
30 April 1997, page 11
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Mary becomes the Mother of all disciples
Jesus’ words, “Behold, your son”, effect what they express, making Mary the mother of John and of all the disciples destined to receive the gift of divine grace.
4. On the Cross Jesus did not proclaim Mary’s universal motherhood formally, but established a concrete maternal relationship between her and the beloved disciple. In the Lord’s choice we can see his concern that this motherhood should not be interpreted in a vague way, but should point to Mary’s intense, personal relationship with individual Christians.
May each one of us, precisely through the concrete reality of Mary’s universal motherhood, fully acknowledge her as our own Mother, and trustingly commend ourselves to her maternal love.
Source: https://www.ewtn.com/library/PAPALDOC/JP2BVM49.HTM
An assumption read back into Christ's words, which He does not say.
Rome just made this up!
The text does NOT say this at ALL!
No. Actually it shows us that Christ appointed John to take physical and financial care of Mary as He was leaving for a time. Nothing more. Most of us make provisions for our parents/children in the event of our death. Even on the cross Christ was supremely in control and took care of every detail.