Can't companies and products simply hit a natural limit? There are inherent limitations to any given "thing" and perhaps Twitter has found its own terminal velocity.
Certainly they can. The issue is that the stockholders had previously believed that this limit was much much higher. They also, to some degree, still believe that (And maybe they are right. Maybe not. It's hard to say!). So if you are the management team at twitter and your stockholders think you could be doing better you are at constant risk of getting fired.