God is still faithful. There are still the covenantal promises. There is still the preservation of the Messianic line because He who promised, He who covenanted, must be faithful.
The Promise Land's true value is in the gift of Jesus who will provide His blood and very life to endow all people with forgiveness and everlasting life for His children.
The LORD God shows His favor and presence with Joshua in the parting of the Jordan and other signs which suggest Joshua is the new Moses… but he is not!
Isaiah finds himself in the presence of the living God—the Holy One. This is a terrifying situation because Isaiah knows full well that the unholy cannot endure the presence of the Holy.
Thus, the people weep and mourn, but they are told to cease with their mourning and rejoice and celebrate as a festival without worry or care, for the day is holy to the LORD who is their refuge and strength.