noun
doctrinaire; armchair theorist
Refers to someone who insists on abstract theories without regard for practical realities. Often carries a critical or dismissive tone.
彼は空論家で、現場のことは何も知らない。
He's an armchair theorist who knows nothing about what happens on the ground.
机上論 is the abstract theory itself, whereas 空論家 is the person who advocates such theories.
Compound of 空論 (abstract or empty theory) and 家 (person who does or specializes in something).