Is eating on the floor compulsory or optional – Sunnat or Mustahab?

Answer

It appears that you are ignorant of the meaning of Sunnat and Mustahab. Sunnat and Mustahab do not mean 'not compulsory' for practical purposes. Eating on the floor is compulsory. It is haraam to emulate the futile and specific styles of the kuffaar – styles which displace the Sunnah style.