I had a conversation with someone about this rule the other day, and the result was that when coming from a writing standpoint, the rule makes complete sense.
Putting aside the fact that people would (and probably still do) abuse being in multiple applicable families at once by permission passing and/or avoiding, being in more than two (and maybe even with the exception of a third one) families make no sense whatsoever.
From an OOC standpoint, most families are made by people are a group of friends OOCly. There's nothing wrong whatsoever with wanting to interact with your friends on SRP, but that doesn't require making a family. And then people will be in many of these family-friend groups at once, leading up to these 7-inital RPNames. To say that being in multiple families at once is 'crucial to lore' is a complete lie, because if anything, it would just ruin the lore of every single family that a person is apart of. And I get that adopting is a thing, but in most cases it's just 18 year olds getting "married" and adopting other 18 year olds.
Don't get me wrong, I was once like this as well. Some folk have seen me walking around as Alejandro M. J. R. S. B. Concord (or something like that) way back when. To think that you need to have every single last name of cousins, godparents, and family friends as a last name is ridiculous, to say the absolute least. At most you'd need really is two. In the context of a child of divorced parents, a birth last name and the new given last name by remarry. You wouldn't keep the mother's maiden name before she married the first time, because that maiden name is gone when she is married and has the kid in her first marriage.
Pardon me if this was a bit hard to read, but I did my best to get my point across. Thank you for your time.