It's unlikely to be a "hobby" for most people. For the same reason that going to work isn't a hobby for most people.Can you stop with your stupid piece of reasoning?
Why can't it still be a hobby if you have 10 GPUs?
Hobbies tend to be something you are interested in, get enjoyment from - and there are two crucial differences that I see. 1) For most of us hobbies tend to cost money. People spend thousands on their hobbies - correct - but they don't tend to expect any ROI for whatever they spend. 2) If mining stopped being profitable the vast majority would ditch this "hobby" quicker than you can say "100 GTX 1060s for sale".
This is a money-making enterprise pure and simple.
And no, the vast majority of gamers wouldn't spend tens of thousands on GPUs. Don't be silly. There's only one Kaap.