Also from a license negotiation perspective, gives the buying company the option to threaten to self-host and/or fork. Even if they never do (and I'm sure the source company is very careful to balance the value story), it can act as a ceiling for rate increases or other annoying business practices.

Why would a company ever open-source their product then? Giving up that complete leverage can be a selling point during the purchasing process, making buyers more comfortable that they won't be (completely) locked in, and be a net positive on revenue through faster sales.