I think if Sailor Moon had ended on just the Dark Kingdom arc it would probably be in the same boat as other more obscure Mahou Shojo like Wedding Peach or Tokyo Mew Mew.
It would have a cult following and would probably be decently well known among Magical Girl fans, but it wouldn't be the influential worldwide phenomana, Shojo DBZ that is it is.
It also probably wouldn't have resonated much with the queer community or held up as an LGBT trailblazer and icon, even with Kunzite and Zoicite. A lot of Sailor Moon's enduring strength is not just because of it's length but also thanks to it being one of the first queer-friendly children's anime that young queer folk in the West grew up on. I've seen many a tumblr post and article from gay individual reiterating just how important Haruka and Michiru's character and relationship was in realizing they were queer. The Outers, Haruka and Michiru especially are some of the most popular characters in the franchise and a Sailor Moon that never got around to introducing them to the cast would've definitely been a net loss for the IP's enduring strength.
Possibly, but perhaps not nearly on the same level. I say that due to the fact that the general consensus I've seen is that S was the most popular season and greatly increased the show's appeal for many people. I've met many that aren't even into the series as a whole but still appreciate it for S. I know that S is the reason I rank the show among my most favorite anime and without it, it'd still be good but not quite the masterpiece S helped elevate it to. That said, I still love the first season and rank it highly as well, but I would be sad if nothing came after it since it'd be so much to miss out on, the subsequent seasons really add to the story and make it more memorable overall. I think it'd probably be more of a cult following instead of having the mainstream recognition and renowned status it has now.
Back in the day anime under 30 episodes was quite rare. Sailor Moon would have still been popular because of tokusatsu element involving girls. And peolle would have still measure and compare with it, even if it was just 46 episodes. The reason Wedding Peach etc are somewhat obscure is because they didn't bring anything new to the table. I love Wedding Peach, but it was created to mimic Sailor Moon down to the character design.