As far as I can tell, every conference is now either a "mid-major" or a "major". There is no term in use for just bad conferences (at least not on television), which of course make up over half of Division I. The term frankly sucked to begin with, but at least, say, three years ago you vaguely knew what it meant. Well, actually, thats not even true, since some people used "mid-major" to describe teams too, even teams that legitimately did not play in mythical "mid-major" conferences. Gonzaga, before the WCC got good, is a perfect example.
Basically, it adequately described the Missouri Valley and the old WAC and C-USA (and sometimes the MAC) and teams therein with some accuracy. Now? Just forget about it. The whole point was the "mid" part. Without a bottom, there can be no middle.