DeathAndTaxes is correct, of course.  A much more complicated estimate of the future difficulty is here.

There's also the fact that the price of Bitcoin itself is volatile -- no one knows for sure what the future price will be.

But, yes, especially if you started much earlier and even if you started now, if you play your cards right, it can be a pretty sweet gig.  But of course do not under any circumstances invest anything more than you can afford to lose.  Essentially there is a lot of risk with the potential for a great reward or a substantial loss.