The number of words you have to know to learn a language properly is why it's a daunting proposition to me and why I've stuck with the two I grew up with.
There are 8760 hours in a year. Even if you somehow managed to learn a word an hour, that's still multiple years to get to 20-40k words.
I think we take for granted how many years it takes for kids to learn a language to adult level when we say "kids have an easy time learning languages". It may be easy in that it doesn't require as much conscious study as adults do, but it isn't quick either. Granted mental maturation messes up the time lines, but I think it's hard to argue that it doesn't still take many years.
So depending on what motivates you the time commitment needed at a minimum can be heartening (I'm taking years because there's lots to learn) or disheartening (This is a multi-year endeavor that I'll have to focus on when I could be doing something else).
EDIT: Though reading up on what B2 is, it is a much less daunting proposition, but even C2 doesn't sound like native speaker level, just able to communicate effectively level.