No one is suggesting removing the word mother altogether. If someone is a mother then they are a mother. But not every person who is a mother has given birth (adoptive mothers, mothers who are partnered with child bearers) and not everyone who gives birth is a mother (trans men who will be the baby's father).

The word mother never was a problem with adoptive mothers or step-mothers. It shouldn't be for pregnant trans-men. When they are pregnant they do have the mother role as there is no manly masculine way to be pregnant. Later they can have more fatherly role, whatever that means.