What is wrong with Ricardian Economics?

Okay I am by no means an expert in economics (Im only 16) but I read "On the Principles of Political Economy and Taxation" and "Essay on the Influence of a Low Price of Corn on the Profits of Stock" and I really think that if we used these methods today we wouldnt be in all the crap we are in. Like take comparative economics, if African countries produced whatever they specialize in, instead of all producing on cash crop like sugar, then they would all become first world nations in a matter of a few decades. Also, the part about how welfare, instead of workfare, would encourage people to become lazy and dependent on others for their livelihood. Also, why are some politicians pushing for a fair tax on consumption, wouldnt a real estate tax be more prudent? It seems more efficient and more fixed to tax based on land value than how much you consume. What if no one bought anything for like 2 years, then the gov't would have punitive income for those 2 years? Im just trying to understand why we stepped away from this system, and those of John Stuart Mill and Adam Smith. Thanks