Purposefully didn't focus on "whose fault it is". A long chain of bad decisions led to this situation. Twitter's board shares a fair amount of the blame too, for example. So does Jack Dorsey for not giving twitter much attention during his reign as CEO.
Wait, why does Jack Dorsey deserve blame? He took a failing company and conned an idiot into paying 2.5x market value for it. Measured by shareholder value, he's one of the most brilliant CEOs in history.
15 years ago it wasn't clear whether Facebook or Twitter was going to be the winner (or if they'd both just be another MySpace).
Jack is responsible for Twitter losing.
It always felt like Twitter co didn't actually understand what Twitter was and almost all of the product innovation came from the community rather than twitter itself.
Twitter's only failure is that it never returned a profit. When you look at the social landscape, it's hard to argue anyone has succeeded other than Facebook.
Twitter's product is great and it didn't need to be as large as Facebook. It shouldn't need to twist itself for the greatest common denominator users who required immediate gratification via algorithmic feeds and short memes.
TikTok is private, but is said to be losing money. Snap is in the same boat as Twitter as being largely unprofitable. It is unclear if Twitch is profitable either.
Many of the worst decisions have their fingerprints all over it.