I'm going to be the Debbie Downer here but I don't believe in her brand in the long term. It's already barely making any noise anymore, purely nostalgia based, has zero world building and the whole scarcity thing is not a differentiating factor on the market. Good for them if they want to be able to add a "sold out" label on their website and pretend this venture is a success but that's about it.
She chose to go with very restricted marketing and let the product speak, except the market is full of Phoebe-adjacents who found their customer base during her absence. Now she's just one of many, and I'm pretty sure her customer base is full of Céline nostalgics (who also shop at the Phoebe-adjacent brands). After having been gone for so long, you need to let people back into your world. Almost reintroduce them to it and show them why they missed you somehow. Instead, she chose to present everything in the driest way possible.
Rome wasn't built in a day for sure, but she's not a new designer who just popped up. She benefitted from months, years even, of anticipation. Everybody in the industry was dying to get the opportunity to work with her on that project. She ended up opting for a rather wonky hiring strategy, and the reality was that the launch was okay, and from the second drop onwards it's been quieter. Then came along the comments on sizing issues, etc.
She needs to focus on world-building. it doesn't have to be a show per se, but some kind of presence that will grab people for good. Because you can't survive on nostalgia forever. Especially when you're not that unique anymore.
I wish her well, yet I can't help but wonder if maybe this launch came too late? Look at NG, post-Balenciaga he seriously considered launching his eponymous label. He even mentioned it on a French TV show back in 2016. But now it seems that it's not the case anymore. Perhaps he realised there's so much time that can pass before your brightest days become a fond memory but are not enough to build a business on.
Is it perhaps, very much like what Lola said? She doesn't need buzz or hype or noise to move the product. She has her core customer that isn't necessarily interested in any of that. They know where to find her and will seek out the outlets in which they can purchase in person. Simple as that.
I think we have become so accustomed to everything being so loud and accessible, in a way, that the barometer of success has been skewed in our minds.