Notice here that this is all related to "the last days". Scripture says the last days is the time period between the first coming and the second coming of Christ, so why do you try to apply this passage to a time after His return?
Don't confuse the last days with the Last Day. The two are not the same thing.
Isaiah 2:2
Now it will come about that
In the last days
The mountain of the house of the Lord
Will be established as the chief of the mountains,
And will be raised above the hills;
And all the nations will stream to it.
The above passage directly contradicts the Amillennial position.
1. A chief of the mountains implies subject mountains.
2. Raising a mountain above hills implies that hills exist.
3. Since the nations stream to the mountain of the Lord, then the nations exist as separate entities.
Isaiah employs the literary device of parallelism to draw a vivid comparison between mountains and nations. In this metaphorical landscape, the tallest mountain symbolizes the nation endowed with the highest authority, standing majestically above all others. Surrounding this lofty peak are the hills, representing national authorities that exist in a subordinate relationship to this supreme power. It is to the great mountain, the pinnacle of governance, that the nations turn, seeking guidance and wisdom in their endeavors.
This hierarchy remains valid only as long as the first earth and the first heaven persist in their existence.