Isaiah clearly understands that there is a destruction coming, and so he asks Jehovah how long it will last. The Lord declares that it will continue until the cities are wasted. That isn’t a time period, but rather an event. It continues the gloomy forecast we saw in the last set of Isaiah chapters.
In this case, there is a turning to the hope at the end of the destruction. Some shall return. That it is a tenth, the Lord’s portion, is symbolic of the righteousness of those who will be allowed to return. Returning the agricultural imagery that began with burning stubble, Jehovah likens Israel to an oak and a terebinth. Both of those trees might be cut to a stump, but will grow new life from that stump. Thus, the holy seed will emerge from the destroyed stump of Israel.