The VCs at this point would be happy with a 3-4x return, because the risk is minimal - companies at this level of maturity, profitability, market dominance, and growth are highly unlikely to fail. So, if they picked up (for arguments sake) 25% of the company, giving it pre-money valuation of $800mm, all they really need to do over the next 3-4 years is build an $3.2B company, which, given 1Password's dominance/quality of product - should be relatively straightforward.
Their killer organic entry is: "Everyone" is already using them for personal password management, which means cost of training/installation/use is trivial to add the Enterprise element.
As a personal user, I consider 1Password the GitHub of password management - sure, there are lots of GitHub competitors, and you can roll your own - but, when there is one product that has completely nailed it - why bother going with anyone else.