Isaiah notes the extreme pride of the people of Ephraim and Samaria. They are so certain of their own strength in standing against the Assyrians that they disdain the prophecies of destruction. The are saying "so what if our buildings are torn down, we will build better buildings to replace them (the reference to "hewn stone".)" They say, "what if the sycamores are cut down, we will replace them with cedars (a more commercially important wood.)"
This arrogance and unrepentance on their part is the cause of the punishing justice of the Lord as marked with the "Therefore" at the beginning of the next verse.
Translation note: The Book of Mormon renders "inhabitants" (plural) where the KJV has "inhabitant" (singular) for verse 9. Tvedtnes notes that the Masoretic text has the singular form, while the Septuagint has the plural. He notes that "there is a great likelihood that the original text had an abbreviated w-yws(h), which could have been read as either singular or plural (Tvedtnes, 1981, p.47).
In this case, the context of the stanza is a focus on the people, and in the context of that focus, inhabitants in the plural would appear the better translation. As a singular, it might refer to a single person, but doing so would most likely refer to a king as representative of the people. That would create tension with the poetic divisions of these stanzas. The plural would appear to be the better reading.