Why can’t she be a Skywalker? I agree that it’s a bit of ham-fisted, predictable writing, and a somewhat odd choice for Rey the character considering her closer relationship to members of the Organa-Solo branch of the family, but given Terrio’s hangup with ‘dynasties’ I can see why they went with her being adopted into Luke’s family specifically, and it doesn’t strike me as being massively implausible or anything.
Not to mention, it makes “Skywalker” the last spoken word of the saga.