deteriorate

abate, de-escalate, diminish, downsize, dwindle, recede, wane

break down, corrupt, decompose, degrade, dilapidate, disintegrate, molder, putrefy

sour, spoil

lessen, lower, reduce

debilitate, undermine

droop, fail, fall, flag, lag, languish, run down, sag, slip, waste (away), weaken, wilt

befoul, begrime, contaminate, defile, dilute, dirty, pollute, taint, thin, water down, weaken

descend

disgrace, dishonor, humble, humiliate, shame, take down

blemish, damage, deface, destroy, flaw, harm, hurt, impair, mar, ruin, spoil, stain, tarnish, wreck

depreciate, downgrade