Agile is better than nothing, which is what most teams would/did have as an alternative.
Pretty sure having motivated people who find the work kinda fun destroys agile/any other methodology. When people wake up excited to pull down tickets or just get shit done, you're doing business right.
What open-source projects in particular do you have in mind? And what do they run on, exactly? And what would this model look like if transplanted into a business setting?
I can see some aspects of agile project management in open-source projects. One, transparency/visibility. Two, working software over documentation. Three, customer feedback. In fact, some large open-source projects have introduced channels for early interactions with customers (i.e. developers) — for example, the RFCs initiatives of React or Lit, or community engagement in various html or css work groups.
I do not know how well developers on open-source projects coordinate/communicate between each other. Or how work gets prioritised. I've certainly seen a fair number of failures in that aspect of open-source projects.
Everything else seems incidental
Re: that fun thing, yes, but most projects aren't fun on a daily basis.
In any case, what I originally meant was that agile is not perfect, but if you've seen startups who don't know any development workflow (ie neither agile nor cascade), software dev grinds to a halt really quickly as they try to reinvent an entire field of management.
Treat it like a normal job. Often no "management" is needed. Get done what you can, talk to you tomorrow.