Why do many theorists doubt that genres exist at all?

Because the definitions are fluid at best, meaningless at worst.  

A genre which is derived from Latin “genus” is a style or kind of music, writing, motion pictures, or other media. A few genres are utilized in many types of workmanship or correspondence. For instance, the genres adventure and sci-fi are utilized in writing, motion pictures, and TV.

answer; i believe the correct answer is (117);

imperial crisis leads to civil war.

roman forces withdraw from britain as goths attack rome.

odoacer topples romulus augustus

huns attack the empire and drive german peoples ahead of them.