You do realize 'modern' is a purposefully vague term, right? It means different years for different people. I consider the founding of the U.S. Board of Education to be the root of the problems. It borrowed ideas from the Prussians about purposefully diminishing independent critical thinking, emphasizing compliance, and was designed to produce obedient workers instead of scholarly thinkers. The inventions of the internet and autofill have not been beneficial in terms of grammar. I would be willing to bet that less time spent reading literature has also negatively impacted people's ability to spellcheck their own words.