David Field has been imagining a world where the new covenant is co-terminous with eternal decretal election and seeing what happens, here, here and here. It's not pretty.
Particularly worth some thought is what happens when you try to move from the (alleged) unbreakability of the new covenant to antipaedobaptism:
You'd have to think that you could identify the big-E Elect by their faith. But that would exclude the possibility of false faith or temporary faith. And since you can't know for sure that someone's faith isn't false faith or temporary faith then you'd never baptise anyone at all.
If the response is, "we're not claiming to baptize the Elect, only those who look like the Elect because they have faith" then you've just separated out baptism from the New Covenant and said that baptism is for "those who look like the Elect to us".
Which is fine, because that's what paedobaptists / covenantalists are claiming: that we operate at the level of the "look like Elect to us" and that's how God intends it to be.
Mind you, if baptism is then the initiation rite for the New Covenant then you've just said that there are people who rightly receive the New Covenant sign but who are not big-E Elect. But if they rightly receive the New Covenant initiation rite then they break the New Covenant then the New Covenant is breakable.
So if you want to argue that everyone in the New Covenant is a big-B believer (decretally Elect) and yet that we rightly give the New Covenant initiation rite to those little-b believers (non-Elect, those with false faith / temporary faith etc) then you must deny that the New covenant initiation rite actually initiates people into the New Covenant. That is, that the baptism of the non-Elect isn't baptism, it's just "getting them wet". And the NT evidence for that is what precisely? Non-existent, that's what.