What is the connection of an abundance of milk to eating butter and honey]?
Rashi: The two Tzon will produce so much milk that people will belittle the milk, and eat only butter, which is the fat of the milk. This is a great tiding, that they will have sustenance after all the desolation. Radak - they will also have honey - "Eretz Zavas Chalav u'Devash" (Shemos 3:8).
Vilna Gaon: Agados (not-Halachic teachings) are compared to honey, and Halachos to butter, which is very fat, and one gets it only through great exertion. When one learns much, he can reach the correct Halachah.
Malbim (22-23): Milk does not last 1 . There will be so much (more than they can drink), so they will need to make butter. They cannot sell milk for business. They will need to eat the butter, for the vineyards will be desolate (23), and all the more so grain fields 2 .
Rambam (Hilchos De'os 4:9): One should not drink milk 24 hours after it was taken from the cow (i.e. without refrigeration - PF).
This is unlike Rashi (25), who says that they will abandon the vineyards, but work the grain fields, for grain is essential [for life]! Perhaps Malbim holds that people prefer to work vineyards, for grapes are more valuable. (PF)
Who will remain in the land?
Rashi: The Tzadikim who escape Sancheriv's sword.
Radak: It is in the days of Chizkiyah, a minority who were not exiled. They will have great satiation.
