You can’t have it both ways, you can’t say the covenant ended at the cross then argue Galatians 4:25 is referring to people who are under that covenant in 48AD.
Wow! The confusion here is failing to distinguish between
the legal establishment of a covenant before God and
people continuing to live under a covenant "they wrongly believe is still in force."
Yes, the Old Covenant officially ended at the cross. Christ established the New Covenant through His blood (Luke 22:20; Hebrews 8:13; Hebrews 9:15). God’s covenantal judgment had already shifted. The Old Covenant was
obsolete in God’s eyes, even though its earthly structures and unbelieving adherents continued in the New Testament period.
Galatians 4:25 is not saying the Old Covenant was still valid in 48 AD like you think! Paul is identifying
those who remained attached to the Old Covenant system and were still in bondage to it because of their unbelief. Their
continued rejection of Christ placed them in the condition of slavery represented by Hagar.
There is a difference between saying, “This covenant is still active before God,” and saying, “There are people who are still clinging to a covenant that God has already fulfilled and set aside.” A person can remain trapped in an expired system, but that does not make the system valid again. Nor your 70AD doctrine!
The same principle applies today: people can reject the New Covenant and attempt to establish their own righteousness through law, but their rejection does not resurrect the Old Covenant. Hebrews is clear: “In that he saith, A new covenant, he hath made the first old. Now that which decayeth and waxeth old is ready to vanish away” (Hebrews 8:13).
The mistake is confusing
the existence of people still in covenantal blindness with
the continued existence of the covenant itself. Paul was describing their spiritual condition, not declaring that the Old Covenant remained in force after the cross.
Selah!