The vocabulary like 現金 is something you will learn at some point anyway in my impression.
But I really think that the vocabulary used in WaniKani is very useful,
not only for advanced learners but also for beginners at some point.
It would just really devaluate the whole thing if it would be blown up to accommodate too much basic Japanese.
I came to realize that you can never know enough vocabulary, in the end it is the most important part of Japanese. If there would be a way to learn vocabulary easily like WankiKani I would definitely pay for it, making Anki deks is very tiring and it also does not work very well because you don’t need to type in the answer.
In my first Japanese lesson I couldn’t say much except 残酷 (because of watching Neon Genesis Evangelion) and the reaction of the teacher was very funny. 
But it does not matter, recently 15 years later I finally heard that word in a TV drama, so it was not for nothing after all and you don’t forget the vocabulary you learned in WaniKani anymore I think.
(If your girlfriend is Japanese it is better not to believe everything unquestioned she says about studying Japanese)