Hard to tell what makes a word unfashionable. Words are tools. A well formed word carves ambiguity at the joint, reduces uncertainty to a minimum, makes meaning as clear as a pane of glass. Why would one ever lay a word down, unless a very much better one presents itself?
Equality has retained its utility unchanged for two centuries. Ability as we have already seen has gained in popularity: perhaps a fanfare for the common man, a democratically acceptable version of “clever”.
Two words stood high and proud in 1800 judgment and wisdom. They have been in gradual decline ever since. Ability surpassed wisdom by 1907, and surpassed judgment by 1937.
Perhaps we should not read too much into this. Seems odd, though. It might account for us being more vulnerable to long cycle risky events, like bank failures, financial crashes, credit crunches, that sort of thing. Just a hypothesis.