It seems like the worm code/the replicated code only really attacks stuff on site. But leaking credentials (and obviously people reuse passwords across sites) could be sooo much worse.
If an attacker wanted passwords en masse they could inject fake login forms and try to simulate focus and typing, but that chain is brittle across browsers, easy to detect and far lower yield than stealing session tokens or planting persistent XSS. Defenders should assume autofill will be targeted and raise the bar with HttpOnly cookies, SameSite=strict where practical, multifactor auth, strict Content Security Policy plus Subresource Integrity, and client side detection that reports unexpected DOM mutations.