If X loses any of these:

1. Profit: X fails to get enough advertisers or it gets too expensive to maintain (e.g., from hosting more videos).

2. Institutions: X is used by large institutions, like governments, and influential figures to communicate with the public. Instagram, YouTube, Facebook, and other platforms can be used in a similar way, but X is the main platform for updates and announcements, so it seems. If it fails to maintain this status, then it could quickly lost relevance.

A lot of celebrities and influencial organizations moving away from it / A competitor gradually growing / months/years of mismanagement / adoption of a form of widespread censorship

Too many advertisers jumping off and the platform just spirals down financially.

Competitors getting better than X.

also, spambots/junk ads are near a critical level of prevalence that could push a significant portion of casual users off the site

a real, major, multiple-day degradation of service would probably do it