It is true that fats are among the most calorie dense food in existence but that doesn't make them necessarily 'practical'. If animal fats have lots of external costs (in terms of environmental damage) then they aren't very practical. The costs are just not well understood.
As for the spinach straw man argument...I don't think anyone is suggesting that you eat 10 kilos of spinach, that would be extremely unhealthy. I think the OP was suggesting simply that calories come mostly from plants. This includes nuts (peanut butter is very high calorie), fruits (also high calorie) and vegetables.
If you allow "plants", then yes- starchy staples, nuts, fruits all have you covered on calories. It's up to the OP though to clarify whether he meant plants or celery.
As for the spinach straw man argument...I don't think anyone is suggesting that you eat 10 kilos of spinach, that would be extremely unhealthy. I think the OP was suggesting simply that calories come mostly from plants. This includes nuts (peanut butter is very high calorie), fruits (also high calorie) and vegetables.