Do you mean that the sacrifice needs to cover the time range in which saving was needed? I don’t see how it would be needed for ages in which saving wasn’t needed. Angels existed before man, and seem to be everlasting, so wouldn’t they “cover all bases”?
In the old testament, we can see that God established covenants with humanity. Whenever humanity would fail to live up to their end of the deal and would break the covenant, their relationship with with God would be fractured.
Each time humanity would turn away from God, God would establish a new covenant with them. These covenants were not eternal though. Each time humanity would fall away, they were without a relationship with God until a new covenant was established.
In the new testament, we see the establishment of the new and eternal covenant in the blood of our Lord Jesus Christ.
Because Jesus is both fully human and fully divine, His self sacrifice on the cross establishes a new, perfect, and eternal covenant with God that is universal. All the previous covenants were with specific groups of people: One man, One family, One nation.
With Jesus, the eternal covenant is established on the behalf of all of humanity. His divinity allows for the covenant and salvation to cover the entire world: past, present, and future.
The old covenants were like a reset button, but they did not help those who lived in the times between the fracturing of one covenant, and the establishment of the next one. With the establishing of the new and eternal covenant, Jesus allowed all of humanity, past, present, and future, to have the opportunity to share in His salvic action.
Angels are creatures; they are neither eternal or divine. Because of this, any sacrifice offered by an angel or covenant established with an angel would not have an eternal, unbreakable effect. Any sacrifice or covenant established by an angel on behalf of humanity would take place at a certain point in time and would only continue until it was fractured again by humanity. It would also not have the power to reach back in time to provide salvation for those in the past, like Mary.
It is also important to see that all of the covenants are established between God and humanity. An angel is not human and has no human nature. They are pure spirit. Because of this, they cannot establish a covenant on behalf of humanity.
Through the Incarnation, Jesus has the unique ability to establish a covenant on behalf of both God and all of Humanity. Through the establishment of the Eucharist, we as humans of all ages are able to participate in His one and eternal sacrifice at every mass. No matter how many times we fall away, we can renew our relationship with God through the sacrament of confession, and again participate in the eternal covenant.