Sort of. In practice, taking on an unmaintained library yourself (whether as a public project or just internally) means taking on unknown amounts of technical debt. ("Legacy code" can IMO usefully be approximated to "code dumped on you with unknown technical debt involved".) It might be lovely, it might be a goddamned nightmare.
Is your hypothetical nightmare worse than the nightmares created by the choices you have with an abandoned closed library? It pretty much boils down to:
a). Doing nothing and living with a buggy closed library you can't fix at all, at unknown cost, placing your business at risk?
b). Being forced to migrate entirely to a new library with all the "technical debt" that entails, at unknown cost, placing your business at risk?
Those are just about the only two choices with a closed source library (aka binary blob), commercial or not. I could add a few more extreme cases:
c). Reverse engineer the closed library and write your own code
d). Sue the vendor for support if the contract or license gives you a toehold.
That's about it.
Having the source available gives you more choices. More choices lets you manage the risk more adroitly. Having source available means you can fix things well enough to live with what you have while you migrate to something else at a pace of your own choosing, with risks and cost known and controlled by you. Having source available means you can weigh the cost between migration, short-term internal patching, long-term internal adoption, hiring a contractor, resurrecting the project and building a community etc. Having more options seems a superior situation to me, and source available gives you those options.
In fact, if you look back at the genesis of FLOSS, the whole point was that source gives you the option of fixing problems yourself rather than being at the mercy of a greedy, irresponsible (or no longer existing) vendor.
You conveniently left the time others did your work for you at little or no cost to you out of your technical debt calculations. That's a gift you're not entitled to in perpetuity and you should always be prepared to bear the cost yourself should the situation arise.