Paul decries the fact that the promises have been voided and replaced.
This has been done by those in Judaism who oppose him. Indeed, it is what Judaism did as he knew it and grew up in it. This statement, this objection, by Paul, is the core of his reformation. The promises to Abraham do not 'need' the law to be fulfilled and they were offered to the nations before the law came. The law is what Judaizers major on, and he doesn't mean the core ethical commands. He means the institution, the dietary and ceremonial laws, etc.
The Gospel is what was promised to Abraham! That is why everyone who believes is a 'son of Abraham' who had faith.
Notice there is nothing about the land or theocracy. That's obviously not what was meant. what was meant was justification from our sins--forgiveness! This is now (re-) offered apart from the law to all mankind, race-kind, gender-kind, class-kind.
The best commentary on Paul is--Paul in his 'sample' sermon in Acts 13. 'Whatever was promised to the fathers has been fulfilled in the resurrection of Christ'! Why the resurrection? Because it is the proof of the justifying quality of Christ's work, as repeated in Rom 4:25. The proof of justification from our sins is the thing promised to Israel, to Abraham! And to be preached all over the world, to be enjoyed, to ignite fellowship and unity! It is the most amazing news and declaration under heaven! He wanted all Israel to be its missionaries! They would have been forceful in it! But very few, very low %s followed. He still wanted all of his people to be like him, except for the incarceration chains he had to wear at the end, Acts 26. To be in this mission! This was all kickstarted by Pentecost, in which those people returned home from that weekend to 'every nation under heaven' with a forceful message completely different from anything the law sounded like. Now it read: we blew it over there in Israel, even killing Christ. But God forgives that through the same loss of Christ! Everyone there and everyone on earth, therefore, owes God their honor! This is the power of the 'reign' of God which is now here. This message. This message compels all men to live in honor of Christ! How could it not?
Something like that. No wonder Paul was less than interested in 'the law' as such, anymore.