Simplification of languages learned by adults happened mostly before written text become widespread (with the invention of the printing press). Written languages resist to change more than spoken only.
Spoken languages have still changed after widespread written languages. Mandarin is substantially different from the Classical Chinese used in writing until the 20th century. Modern spoken Finnish (puhekieli) is substantially different from written Finnish (kirjakieli).
H and L languages. The divergence between H and L languages (often called disglossia) is a continual process ever since the invention of writing.
And? How is that relevant to whether or not some languages are harder to learn than others?