@Zhorn: the statistics I saw comparing editions up to 4th are pretty dated by now - IIRC they were from 2010 or 2011. BECMI's relative dominance was largely due to receiving largest number of localizations and bulk of its sales were made during TSR era. I'd argue up until 5th edition, there were way more people who transitioned from BECMI to AD&D or only played BECMI, with people who transitioned from AD&D, 3.x. or 4th edition to BECMI being a drop in the ocean.

Now, this was physical books only. Since then, WotC has made D&D's back catalogue more accessible in PDF format. So if BECMI has grown a fat tail due to digital distribution, I wouldn't know.