Given the contracts handed out to similar pitchers such as Wade Davis and Kenly Jansen, I think we'll see closers cross the 20M dollar threshhold within the next year or two, yes.
Again, you're trying to say this for shock value, but in an era where top starters made 20M, top closers got 12-15M, so when top starters make $30M, $20M isn't as unreasonable as you're making it sound.
As a general rule, the league's top closers tend to march lockstep with #3 starters in terms of salary requirements, at least historically