In general anticipating how much expositions may be needed is difficult, but filmmakers usually shoot new takes providing more informations, if they find it necessary after a test screening.
I see what you mean, though the “common practice” I meant is “explaining everything directly” (that is, through expositions), not just “explaining everything”. You say you’re glad they left the Resistence’s origin (for example) unknown, but that’s not working against this practice, it’s just not telling anything at all 😄 (whether this was a good idea or not, it’s another story).
On a side note, while not explaining the origins of the Republic and the FO was probably intentional because unnecessary to understand the overall plot, I think they actually did try to explain the Resistence’s relationship with the Republic, but I’m 99% sure it was then just cut out along with a deleted scene (probably the one involving Leia), so I don’t think it was really an intentional decision from LF.