You might be able to reach similar levels of language proficiency without the courses but the biggest problems are 1) finding hours and hours a day to drill writing characters and other mundane aspects of learning 2) guidance from an instructor who can tell you whether things that look/sound right are. And also I think a university course also gives you a survey of the literature and culture of that language and insight into it that would be hard to acquire on your own.
Also how long have you been studying languages? I ask because I am curious if you think that the web, and later, apps, like Duolingo for example, can make a huge difference versus the pre-web era of language learning. Pre-web, I would agree with anyone that yes it would be extremely difficult to learn a language on your own unless you had traditional schooling. But today there is nothing a traditional course in German could give me that I couldn't get from Deutsche Welle, for example (but I also realize there isn't a 'Deutsche Welle' for every language).
Sorry if this is going further than you wanted, I am just trying to get different perspectives and it seems like you have a good grasp.
I started studying in middle school. I haven't tried the online stuff you're talking about. I'm not sure that they would be just as good.
If while you are there, and are aware of any similar websites like that for any other language I would love to know!
My good friend has a PhD in Chinese, (yes that is actually a thing), and he attests to the fact that he didn't grok the language until his travels and studies there, being immersed.
Now typing this I realize we may be debating across each other, because even if you believe you can learn on your own better, or having structured courses is better, the real answer is none of it sticks unless you fully immerse yourself in it, self-taught or not.