Yhwach doesn't drain the power during the fight, he takes the power back when a Quincy dies, whether it is in battle or old age. Soken Ishida also didn't die when Masaki died, it was Uryu's mother who collapsed and fell into a coma because Yhwach took back the quincy powers from anyone who was not a pure blood Quincy. Masaki was supposed to live like Soken and Ryuken since she was a pure blood Quincy but being bitten by "Whitey" corrupted her.