The oft quotable F.D. Bruner interacting with Matthew 6:19-21 in Matthew: A Commentary. Volume 1: The Christbook, Matthew 1-12
writes (page 321):
Jesus does not quash ambition; he elevates it. The Christian is to be ambitious, passionate, acquisitive, enterprising — for the Father’s approval, for the “well done” of God’s Final Judgment. Thus Jesus’ ethic is not so much ascetic as athletic.
The moth is nature’s corrosion eating away, the rust time’s corrosions, and the thief humanity’s corrosions — and all three together represent the insecurity of life lived for accumulation.