'Rich' originally meant 'powerful' — from PIE *reg- (to rule). Wealth replaced royalty in its meaning.
Having a great deal of money or assets; wealthy; plentiful, abundant; producing abundantly; having fullness of quality.
From Old English 'rīce' meaning 'powerful, mighty, of high rank,' from Proto-Germanic *rīkijaz (powerful, ruler), which was borrowed very early from Celtic *rīgos (king, ruler), from PIE *h₃reǵ- meaning 'to move in a straight line, to direct, to rule.' The same PIE root produced Latin 'rēx' (king), Sanskrit 'rājan' (king), and Irish 'rí' (king). The original meaning was 'powerful, ruling' — the shift from political power to material wealth occurred
'Rich,' 'regal,' 'royal,' 'reign,' 'regent,' 'rex,' and the Indian title 'rajah' all descend from the same PIE root *h₃reǵ- (to rule). But 'rich' entered Germanic through an unusual route — it was borrowed from Celtic into Proto-Germanic before the Germanic languages even separated, making it one of the oldest known Celtic loanwords in English.