I can pretty easily keep a mysql instance running. I cannot easily do that while adding features to it. Scale that up a lot and you can see how Twitter can keep running with much less people while not delivering anything.

The fact that a site that big hasn't broken yet shows how good they actually were and in the end he did fire good engineers while not even tackling Twitter main problems which none were technical. Bots are still rampant, moderation is gone, monetization is non existant and advertisers were scared away.

Unless things change what you're seeing is not a success, it's a zombie.