Watching TV series consistently is proven to improve listening skills and vocabulary. I use to watch a lot when I was younger.
But of course, speaking is also important, watching without being able to speak the words you learn would mean you will forget most of what you have learned. The more you use words that you learn, the more you remember them.
You are too stilted. Please, take it easy, because what he says makes a lot of sense. Every one of us has the ability to use different language registers and gaining more registers only enriches your speech.
Watching long term TV series with many many seasons and many many episodes is actually a great way to not just learn, but acquire a language once you are advanced enough to understand what it is said, or written if you have to resort to reading the subtitles because you don't understand the spoken language at the native fast speaker level.
It matters not if the vocabulary used in the series is outdated or uncommon, what matters the most is three things: that you understand or can infer most of what it is said, that it keeps you engaged and that you listen to thousands of hours of native speech.
In the end, you are going to end up watching many different series, and that's how you erase your memory of the plot :-)