The circumcision was celebrated because this was the first time God shed blood for mankind, which is NOT the same as the incarnation of Theotokos. They are celebrations of different things.
And isn’t this clearly archeologism? “The early Church had a feast, lets replace the later feast with that.”
The circumcision of Christ is still commemorated on this day, it is just not the focus of the Solemnity. When teaching RCIA, I always point out that we also commemorate Christ’s circumcision, which according to Jewish Law was to take place seven days after a boy was born. New Year’s day marks 7 days after Christmas. Both of these mysteries of the incarnation are expressed on this day, the one you stated above, as well as Mary being the Theotokos. Here are the two crucial readings of the day, they express both mysteries:
“When the fullness of time had come, God sent his Son,
born of a woman, born under the law,
to ransom those under the law,
so that we might receive adoption as sons.
As proof that you are sons,
God sent the Spirit of his Son into our hearts,
crying out, “Abba, Father!”
So you are no longer a slave but a son,
and if a son then also an heir, through God.” (Gal 4:4-7)
"The shepherds went in haste to Bethlehem and
found Mary and Joseph,
and the infant lying in the manger.
When they saw this,
they made known the message
that had been told them about this child.
All who heard it were amazed
by what had been told them by the shepherds.
And Mary kept all these things,
reflecting on them in her heart.
Then the shepherds returned,
glorifying and praising God
for all they had heard and seen,
just as it had been told to them.
When eight days were completed for his circumcision,
he was named Jesus, the name given him by the angel
before he was conceived in the womb." (Luke 2:16-21)
unfortunatley most people skip over the circumcision of the Lord, because they don’t hear about it from the pulpit. When I was talking about this Solemnity I made sure to pay attention to this other commemoration.