يَذْهَبُ is a present-tense verb meaning “go away.” الصَّالِحُونَ is a sound masculine plural ending in ون. وَتَبْقَى means “and there remains,” placing two successive conditions in contrast.
In كَحُفَالَةِ الشَّعِيرِ أَوِ التَّمْرِ, كَـ introduces a comparison, “like the remnants of barley or dates.” أَوْ means “or.”
The contrast makes the message clear: the departure of the righteous is followed by a decline in the quality of those who remain.