The phrase originally meant easy to learn with diminishing returns after a certain point, for exactly the reasons you suggest. However, the phrase "steep" implies something hard to do, so the phrase gradually lost that meaning and gained the opposite. I fear the same fate for the word "nonplussed." "Unique" is already half dead and is migrating to a synonym for "unusual." The subjunctive tense is huddled in its bunker somewhere, wishing it were more popular. The pluperfect feels secure now, but its day will come. Its day will come, friend.