I think the mixed historical record of consul republics might be relevant to why a single leader is often a better idea than 2-3.
I'm thinking that a project that starts out with 3 BDFLs (like yours, @albertzeyer) and works fine & builds sth people "love" — it'll continue working fine with those 3 BDFLs.
And a project that starts with just 1 BDFL, and works fine & builds sth people "love", it'll continue working fine with that 1 BDFL.
If, however, replacing one of those 3, or adding another BDLF to a one person project — I'd think that by default, that won't work.
... Unless the new person has worked next to the earlier BDFLs for years, so the previous BDFLs can be ok certain s/he is "the right one"?
Whatever works fine, works fine — the risky thing, is introducing a change (a new person)?
Thing is, with a consul or triumvirate system (you can’t have more than one “dictator”; all of these terms are from Ancient Rome anyway so let’s use them properly), if any disagreement arises between the 2 or 3 leaders, it will develop a new instability where there wasn’t one before. Or else, one of the 2-3 leaders will start to dominate the others. Even without turnover the dictator system is more stable. In fact the only benefit of a consul or triumvirate system might be the stability and continuity under turnover.
It seems that this model is not so common? At least the list on Wikipedia (https://en.wikipedia.org/wiki/Benevolent_dictator_for_life) only lists Django as an example with 2 BDFLs.
I was active in a smaller open source project (http://www.openlierox.net) where it mostly was like that, i.e. we were 3 main developers, and all major design questions were discussed among us. But this example is probably not so representative, as there rarely were further contributions by other people. But anyway, I was very happy with this model, that there is not a single leader, but 3 people.