This is the reason I won't sign up for any new startup ever - it seems that eventually they all get acquired by one of the old guard behemoths anyway.
No matter how virtuous and privacy focused they are, that all gets destroyed when someone buys them.
Personal income information about users is generally 3rd party estimates for ads/marketing industry, but by offering a free Tax service, CK was able gather information on actual income about users, which is extremely valuable data.
This would be very lucrative for data-brokers or sold as targetable and proprietary 'audience data', which can be combined with other data-sources to support marketing activities.
User data is far too important to sell.