Rosalina is still strong without Luma, but not as dominating. For example, Diddy has a much easier time approaching and shield-pressuring a Rosalina without Luma, and Sheik can f-air Rosalina safely without worrying about Luma up-tilt getting in her way. Starbits and Luma Warp, as strong as they are as custom moves, are entirely shut down when Luma is shut down.
The respawn time was upped by 5 seconds, which seems trivial until you consider that it's a 62.5% increase in the total Luma timeout. Suppose that in a normal match Luma goes out of commission 5 times (this is a conservative estimate that will probably rise as people become more accustomed to the character and as people start to use their custom movesets more often). That's a total of 40 seconds of the match during which Luma does not exist. Luma will now not exist for 65 seconds--an additional 25 seconds over the course of a match. Assuming that the average match takes 3 minutes to finish, that's a jump from 22% of the match time to 36% of the match time. Seems much more significant when viewed in these terms.
Consider that a respawn time of 13 seconds makes it far more worthwhile to kill Luma in the first place, and that people will get better at fighting Luma as the metagame develops, and you can up that 36% to 45%. That's almost half the match during which you fight Rosalina only, and not Luma.
With this change, I no longer believe that Rosalina is top tier, though I am aware that she will still have many of the strengths that make her top tier at this time.