Making “all my mountains a way” suggests moving in the high places; being in the mountain of the Lord’s House, or having temples available. It presumes there will come a time when temples become available. It foreshadows our day.
We live at a time when the “mountains” are available to be a way; when walking in those “highways” will make one “exalted.” Nephi saw these things, knew they would come for us, and wrote his record to bear testimony through Isaiah’s words of the unfolding Gospel plan of the latter-days.
Of course, having these things happen requires a gathering.