It's true that modern Japanese is used in most Taiga Dorama but it's pretty cleverly mixed with Archaic Japanese. Basically the general public in Japan now can't follow the storyline if it's entirely in Archaic Japanese...even if they can it's very demanding on the audiences and it becomes too much of an effort for them to stay interested. Often in the dorama when people meet, have particularly scene setter or climactic moment they use the Archaic Japanese to present a real mega sense of authenticity!
There are dorama like Shinsengumi! which use quite a bit of Archaic Japanese but the modern is still incorporated basically so audiences can follow the plot.
Hell, I find it so hard to keep up anyway! I'm asking my mum vocab questions every 2 seconds!!!
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