Therein, lies the problem. You have it fixed in your head what is meant by "few" as being certain number - which you have no idea of what that number is that you can pinpoint from the bible. You can't say 20, 200, 2000, 200,000, 2 million, 20 million, 200 million. Of the Jews, one could reason a third of the population will survive the great tribulation from the text in Zechariah 13:8.
Few in the Isaiah prophecy is a relevant term in regards to the population of the earth before the Great Tribulation begins and after it is over. You don't have an actual number. No-one has - humanly speaking.
So, we don't know exactly what a "few" is, but God has predestined individuals and entire nations to either be saved or not saved - Which has got nothing to do with Calvinism, since Calvin denied the existence of freewill - According to Romans 9:22-23.
This means that some countries would still have more survivors than others, based on the law of average that some people would stand a better chance of surviving than others - Since they are genetically and environmentally predisposed to survive, as opposed to rejecting God's covenant.
So, although there are invariably only a "few" from "every country" it can be as few as 2 people, which means that such a country could never survive in the long term, but they would ultimately lose their sense of national identity by way of assimilation with other countries which are numerically greater.
Thus, entire countries could effectively be extinct in the long term, although there would still be survivors from "every country" in the immediate aftermath of the Tribulation.
Such extinction could still happen without God breaking his promise that "Every country" would survive the Great Tribulation.
Since their survival in the immediate aftermath of the Tribulation (Revelation 7:9) is no guarantee of their long term and eternal survival in the new heaven and earth.
So, God has predestined some countries to be "vessels of wrath" which would be destroyed by the Great Tribulation, but this does not in anyway contradict his promise that there would still be survivors from "Every country" (Revelation 7:9) in the
immediate aftermath of the Tribulation.
Since in the long term, entire countries could still be extinct if they have only 2 survivors.
Also, there is no guarantee of diversity amongst those countries who survive the Tribulation.
Since God promised that "Every nation and tribe" would still survive the Tribulation, but the Bible is no guarantee that there would still be diversity amongst those countries that survive the Tribulation.
Although God has promised the
immediate survival of "Every tribe" - Which are transnational diasporas - there is no guarantee that "Every tribe would still be represented within every country" in the immediate aftermath of the Tribulation.