I don't know much about udemy, but this downward trend can be seen in plenty industries. Sadly, this usually turns the products in question into trash as well.
Content theft being involved though is the actual low blow I see regarding this topic. There are a lot of grey areas and loopholes that thieves can use and abuse which makes content creating quite risky at times.
I am not big on these online training markets and not all operate the same I believe, but are there really no better alternatives than udemy? I heard skill share is decent.