I don’t necessarily think that it means that it’s a failure, Apple just cycles through the same several materials over and over again when there’s nothing else to add.
This is not even the first time they have added titanium to a product just to remove it later.
There was a titanium PowerBook G4 in 2001.
It was replaced with an Aluminum PowerBook G4 in 2003.
iPods throughout the 2000s had a stainless steel back, these would all gradually become aluminum by 2012.
Even the iPhone, they just cycle through the materials and never really consider one more premium than the other.
The first iPhone was aluminum and plastic on the back, the next ones were plastic completely on the back, the next ones were glass and stainless steel, the ones after that were glass and aluminum, then they were aluminum with plastic antenna bands, then back to Glass and stainless steel, which became glass and aluminum, which became glass and titanium, and what do you know it they’re going back to aluminum again.
Maybe with the recent iPhone 16E coming in black and white, and there being so many complaints about the pro iPhones just coming in different shades of gray, they’re going to have a super colorful pro iPhone line this year and the only way to really do that is to go back to aluminum. Maybe they want to make the screen slightly bigger again and want to offset any additional weight by going back to aluminum.
Maybe they are switching because it’s a more environmentally friendly material.
The cynical (but probably correct) reason is that it’s a question of importance versus expense and aluminum is significantly cheaper for Apple and they know that most customers do not care.