I have never really thought about that once. But apparently if mining stops, how will transactions be confirmed? Well, it is good the possibility of something like that happening is zero.
This is only the start of battle and hope it won't get into a matter of losing it all of a sudden.
But being illegal, I don't think so. Because more countries are making bitcoin legal and it could be also into mining of course. Maybe it happens become illegal, only to the country that bitcoin isn't established yet.