A feature of what service though?

The OS? iCloud keychain does this, it's not a compelling offering though if you need to use any other OS.

Something like Google? Not sure I'd want to risk my Google account ever getting locked and loosing access to all my other accounts.

I'm not sure what that leaves.

The browser is the obvious option. Firefox and Chrome both implement ways to save passwords in the browsers. I believe Firefox has a service to sync them, Chrome may too (I don't use them, so I don't know).

They could reasonably tie in to whatever Office-suite you use (GSuite, Office 365).

In the enterprise, it could be part of a larger "credential management suite" product managed by security. Allow syncing and auditing of credentials, like "when was the last time this cred was changed?" with some kind of automation to generate and push a new credential when need be.

From the outside looking in, a basic credential manager doesn't seem complex enough to be a standalone product.

Is that a large enough market to be a sustainable, profitable business?

I would think so, on the business side of things. I'm not entirely sure what we pay for 1Password because we pay it without question tbh. We have a fair few subscriptions but 1Password would be up there with the indispensable ones.