I think the easiest way to manage it is essentially to do nothing. Accept open source contributions without a contributor license agreement and their copyright locks in future maintainers, yourself included. Extricating those contributions eventually becomes impossible without a cleanroom rewrite that is usually economically impractical and way too risky to a business with revenue.
This requires a copyleft license, and can be bypassed if all contributors agree to sign away their code to a company trying to relicense and monetize the code (as the Audacity contributors did for some reason).
