The fact that a remnant still exists gives credibility to the possibility of it returning to its original promise. The addition of many other nations finds in ideal Israel the standard by which all Christian nations should behave.
It isn't clear to me if Peter is talking primarily to his Jewish brothers, or to Christians in general. Certainly, in his time Israel was the ideal nation, it being that no other nations had yet entered into covenant with God, as Israel had. Gentiles were, so to speak, being grafted onto the tree of Israel.
However, once the Roman Empire had converted to Christianity, Gentile nations began their own odyssey with God, and the nation Israel was demoted to being merely an ideal example. I just don't think Peter would've elevated Israel, the nation, so much if it wasn't true that Israel would eventually rise to be more than just a remnant of believers. More, the entire nation could be converted to Christianity, just as many Gentile nations have already done. "The first shall be last."
1 Peter 2.9 But you are a chosen people, a royal priesthood, a holy nation, God’s special possession, that you may declare the praises of him who called you out of darkness into his wonderful light.
God has a future Nation Israel, That's Going To Teach The World How To Behave

:
Scripture clearly teaches "The Elect Remnant" are chosen and elected to salvation through God's "Foreknowledge" and "Added" to the "Church" just as all believers are saved and "Added" to the "Church"
There isn't a future for a "National Ethnic People" as you believe and teach, it's saved church, and unsaved wicked, it's that simple
Then you go further to claim all National Ethnic Israel will be saved

: "Wrong" read verses 7-8 below real slow
"Israel Hath Not Obtained"
"God Has Given Them A Spirit Of Slumber"
"Eyes That Don't See, Ears That Don't Hear"
Romans 11:2-8KJV
2 God hath not cast away
his people which he foreknew. Wot ye not what the scripture saith of Elias? how he maketh intercession to God against Israel saying,
3 Lord, they have killed thy prophets, and digged down thine altars; and I am left alone, and they seek my life.
4 But what saith the answer of God unto him? I have reserved to myself seven thousand men, who have not bowed the knee to the image of Baal.
5 Even so then at this present time also there is
a remnant according to the election of grace.
6 And if by grace, then is it no more of works: otherwise grace is no more grace. But if it be of works, then it is no more grace: otherwise work is no more work.
7 What then?
Israel hath not obtained that which he seeketh for; but
the election hath obtained it, and the rest were blinded.
8 (According as it is written, God hath given them the spirit of slumber, eyes that they should not see, and ears that they should not hear;) unto this day.