What happens if a conference doesn't get enough submissions?
For a conference without integrity, it can of course simply accept whatever washes up on the shore.  

For a conference with integrity, however, there is generally a good deal of flexibility in the structure of a conference program.  Putting together a program is always a delicate balance of talk length vs. number of tracks vs. plenary events like keynotes and panels vs. breaks, lunches, etc.  Thus, if the number of submissions is low, but still enough for a viable meeting, the organizers will simply end up with less papers and a more relaxed meeting pace.  You can easily flex the number of talks up or down by 50% in even a single track conference by playing with the schedule structure.  For a multi-track conference, it's even easier to scale the number of tracks.

If the submissions are very low, to the point where it is impossible to fill even a very relaxed schedule with reasonable talks, that's a different story. It's unlikely to happen all at once, because it would take a remarkable collapse to be down by 50% in a single year. In any case, societies like IEEE or ACM provide a conference with financial insurance, so that it will not be a complete disaster on a fiscal level, at least.  The next year, however, the society may no longer be willing to sponsor the conference...