The story
Probably a calque of a similar formation in a continental West Germanic language, such as Dutch rijkdom (“wealth”) or German Reichtum (“wealth”); equivalent to rich + -dom. Compare Middle English richedom (“wealth, riches”), Old English rīċedōm (“dominion”), Swedish rikedom (“wealth, richness”), and Icelandic ríkidómur (“riches, power”).
Etymology from Wiktionary, CC BY-SA 4.0.