This is less a reason to use Lemmy or MBin over the other specifically: One of the great features of the fediverse is that the content is not siloed off behind one interface. Usage and development can happen on both and any number of other interfaces and all of them will have access to the same content (barring federation issues, but that should become less of an issue as ActivityPub and various interfaces mature).
As for there being enough people to populate interface specific communities/magazines/whatever, you can't take a snapshot of today and project that into the future statically. The fediverse population is still relatively low compared to commercial social networking sites, but there is enough of a core userbase for new people to accrete onto over the course of time. There is a potential future where the user base flips, or doesn't but both Lemmy and MBin have large userbases, or another interface that doesn't even exist yet takes off and becomes larger than both. But it doesn't really matter because all that's happening in those cases is people are being offered different ways of accessing the same content that better match their preference.
Bringing it back to the original point, that the content is not siloed means development on various interfaces can happen concurrently to make things not necessarily better than each other, but more suited to different tastes. You aren't locked into whatever Reddit, or Twitter, or whatever decides the interface should look like.