Same as with OS updates, browser updates, dependencies used by the OS, dependencies used by the browser. Also you can run malicious software such as keyloggers and you're compromised.
That argument doesn't mean E2E (even web based) is snake oil. Browsers just give you more points of failure.
For some companies (eg facebook, google, tiktok) i would be mostly worried about the company itself being untrustworthy. For others I would be mostly worried about the company being vulnerable.
Depends on who is defined as the other end, it may be that the company db is the other end.