What you are missing is that some will leave when it becomes even more unprofitable and the difficulty will drop.

Also, there will always newbies doing it and people that just want to support the network. Extra funds permitting,
I will probably mine the rest of my life. Heck, I've been running 3 Jales till just recently. Now I'm just running the Saturn. You leave out the fact that many have free electricity - kids at their parents house and many in apartments or using the servers at work(which I don't agree with and could do the same myself).