The best way to learn a language is spending a good size chunk of money. People are loss adverse. The need to extract value from the sunk cost is a powerful motivator. That's why in-person classes tend to be more effective than books. A book is always going to be there. Missing a class on the other hand is money down the drain.
I don't think this is true. I've spent lots of money on "necessary equipment" for my hobbies, and that hasn't made me commit more time to them. People buy expensive running shoes because they see themselves as being runners, and then they never put them on.
Your language skills deteriorate without use as well. I used to know German at a passable level at school, I no longer do. I can see patterns ans know some words, but it's definitely heavily degraded.
