I'm still at the beggining of the 10th volume, so I could simply be uninformed (this is not an invitation to tell me spoilers), but the way I've seen people talk about Sunny being fateless doesnt reaply make sense to me.
Often I see people mention that nothing really changed, especially regarding how everyone forgot him, but everyone has pretty similar relationships as before. People saying it made no difference.
First of all, we don't know that. We dont know what would have happened otherwise.
Second of all, I think a lot of us forgets that we don't actually know how fate works, or even what it actually is, un the context of the story. There is a demon of fate and a different demon for destiny, and we still don't know what the distinction between them is.
Often in the story, there comes a point whrn Sunny realises that his initial understanding of something was flawed because he misunderstood it's premise, and many more moments where the way one interprets the same information can lead to vastky different results. Fate remains one of these big nebulous mysteries that we don't know if we're even seeing it from the right angle.
Third of all, the biggest clue we have about how fate works is that it's like a tapestry, similar but in a completely different realm than the tapestries of enchantment Sunny weaves into memories. That is the biggest clue as to why things are developing the way they are. Just like changing a memory's weave, Sunny has plucked out a strand that was originally there, only to replace it with a shadow strand of his own. By stepping back into his old role, he's become that strand himself, preventing the tapestry from unraveling. However, even if it plays the same role, a shadow thread is not the same as an essence thread, and he is not the same as who he used to be.
Perhaps that is a hint towards how he'll change fate, replacing weaver's weaving with his own. Perhaps things would have gone completely differently, in ways we cant even imagine, if he hadn't become fateless.
Regardless, from what I've learned, reading this story, I've come to expect several layers of meaning and development from the crazy stuff that happens to Sunny, as well as how it happens. If it hasnt come up yet, thrn its only a matter of time.