I think one reason it felt like a bubble is in part down to him removing himself from the title picture right at the start. Once they established the TNT title you got the feeling that it was the "Cody" title. It didn't feel like a secondary title, like the Intercontinental title or US title. It didn't have a unique constraint to it, like the old TV title or the Cruiserweight title. It wasn't associated with a particular style, like the X-Division title or the Hardcore title. It just felt like "This is Cody's because he can't have the world title."
This was particularly true at the start, when it went Cody->Brody->Cody in under two months. None of the other titles had changed hands that quickly to that point, and the reigns for the two other singles titles AEW were all in the triple digits prior to that.
They've been running into a similar problem with the TBS title becoming the Jade title until Nyla's angle started.
I also think that if Brodie hadn't passed away it would have felt less like a bubble. Because he disappeared basically right after he lost the belt, it resulted in a disconnect I don't think we'd have felt if Brodie had proceeded to feud with Hangman as was expected. So you had a chunk of time where Cody was feuding with Brodie, with a stable that was basically never on tv (the Factory), and with a bunch of people who mostly weren't even wrestlers (Lambert et al).
I'm not blaming anyone, I'm not saying a particular person made a particular decision or another person was at fault for the booking. I'm not passing a value judgement on Cody or anything. Just trying to pin down why it felt, to some of us, like it was its own bubble.