I'm pretty sure she can step down from being Queen, if she is actually Sailor Cosmos, since there's no indication that she's a queen in the far future.
There's also the likelihood that the millennia long lifespan is about to wear out for Neo-Queen Serenity and she'll kick the bucket of natural causes. It's not clear if this means that Small Lady will have a super-short reign as a result, given that her own lifespan would also be coming near an end, or if the thousand-year timer won't start kicking in until she's nice and grown.
Either way, that's why I've always hated the "extended lifespan" plot point. When dealing with a hereditary line of rule like a kingdom, it introduces all sorts of problems in terms of succession. We already have issues in real life with the Queen of England, and she's living at the outer edges of a human lifespan.
This has me wondering, since they're princesses, they have to get back to their own thrones and become queens, assuming they were crown princesses and heirs apparent to their thrones. If they haven't started families before now, they'd only have a few years to give birth to the next generation.
I personally liked the notion of the Senshi's extended lifespans it fits with them being descended from Gods. Also I think the millennia life span only applies for regular humans I believe Senshi live indefinitely if not killed by an outside force. I mean Sailor Galaxia was alive since the Sailor Wars which I think predated the Silver Millennium and Sailor Lead Crow and Sailor Tin Nyanko were atleast alive from before the Silver Millennium fell to know Phobos and Deimos along with Luna and Artemis respectively.