It doesn’t have to be complicated. Create a quick placement test that gives you an approximate estimate of your current level, then give people the option to skip things straight to Guru if (and only if) they get them right without looking at the lessons first, and only for those first few levels.
This is just one potential idea, but I won’t accept “It’s completely impossible,” since it doesn’t seem to be a problem for any other language school, or any other tool for learning Japanese on the planet.
The problem people have is that if they did retain most of Genki, WaniKani is basically completely useless for the first two months. And there’s no logical reason it should be besides “But SRS!” and “Well if you don’t like it, then too bad!”
If WaniKani can’t think of something, they should just admit on their FAQ, “We’re not smart enough to create a placement test for WaniKani. But if you keep using our system, we do think you’ll really like it eventually!” Which is true, anyone can eventually learn from WaniKani, but you’ll never convince me that someone who’s finished Genki II needs to spend two months getting to level 8–no matter how little they retained.
This is why placement tests exist: so that you can gauge someone’s level in a way that’s difficult to fake.
And if WaniKani phrased it like that, I would understand. But the FAQ doesn’t say “then maybe our program isn’t for you,” the FAQ says “then maybe you don’t have the patience to learn kanji,” which is completely ridiculous.
WaniKani is definitely an amazing program, and an awesome resource. Even at level 10, I feel satisfied enough with the difficulty, even though I still know around 40-50% of the kanji and vocabulary that come up. The problem was the first few levels, where I literally knew things so well I would type them in Japanese and then get momentarily confused when I was asked for the English translation–in my head they were one and the same. Being asked to do this once is bad enough, but four times? Come on. Anyone with two brain cells can tell this isn’t necessary. It’s why every other resource on the Internet offers a placement test of some sort.
WaniKani doesn’t because, well, the only logical reason I can think of is hubris. Their radical system is just so amazing you absolutely must go through it from the beginning no matter what. Don’t get me wrong, it’s decent enough, but it would be much more useful if people could actually use it to learn right from the get-go, instead of just pretending that no one using the system has ever studied Japanese before.