Isaiah takes up the conventions of wisdom literature to illustrate that the Lord of Hosts knew what he was doing in all the complex process of bringing about salvation through judgment.
A true disciple's condition is blessed because there is the expectation of mercy to help, unfettered communion with God, and an honored place in God's house.
Even to the scornful mockers, the message of grace is extended with an invitation to believe in the Lord's foundation stone, reinforced by a warning of the futility of all other refuges.
The disciples of Christ are blessed now, because although their qualities are a despised vulnerability in the world, their approaching inheritance will bring full satisfaction.
Even those who might consider themselves disadvantaged in some way (like the son of the foreigner or the eunuch) are invited to come and gather on God's holy mountain in blessedness.
The persistent Gospel invitation to rest had been rejected by Judah, even by the prophets and priests, with a contemptuous characterization of Isaiah's ministry.