Spitballing, $100M, assuming investors want 20% per annum return and Bitwarden do 50% profit ... they need 24M paying customers. Where are they at now?
If BitWarden can use that $100m to 10x their valuation and then exit (whether that's an acquisition, going public etc.) the investors will have secured a win: if BitWarden's valuation stays where it is and returns ~$10m/year to the investor(s) over the next decade, that's not a great outcome considering the opportunity cost of capital.
Debt equity is the type of financing you're describing: lower risk, lower returns, not particularly exciting and not particularly attractive to investors if they believe the company has substantial upside potential.
Clearly Bitwarden isn't a unicorn, being a smaller entity in a growing market; do VCs really expect a 10x (in couple of years?) from that sort of investment?
So, do you agree with my basic premise that they'll need a whole heap of customers, that they don't seem likely to get, in order to make any dent in the investors hoped for returns?
Every company’s answer to that is also the same “we will target the enterprise”.
They aren’t “doing well” if they still require outside funding.
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.
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.