100% buy in is right.

See https://en.wikipedia.org/wiki/Disagree_and_commit for a list of successful organizations that have adopted a principle meaning that and why they did it.

"Disagree and Commit" in a vacuum is just an authoritarian kludge.

The Amazon principle is "Have Backbone; Disagree and Commit". Most leadership principles in Amazon are in conflict with each other, creating a sort of checks-and-balances situation. But "Disagree and Commit" is so powerful that it needs a check in itself. It doesn't work without "Have Backbone" - but most senior leaders I see who love it never mention that part. They always spell it out as a "follow the orders once they're made" sort of thing.

Disagree and Commit is not about 100% buy-in. It is about having the fortitude to disagree, but once a decision is made, 100% committing to and supporting that decision, despite disagreeing.

There's a lot to like in what you write. Back to the book I referenced above ... disagreeing here has several facets. First you have to be aware you can disagree. You have to be aware you have choice and use it. Disagreement is not a firing offense. Bosses are not entitled. Second, how it's done is important: if I disagree through a defensive behavior (again see book) that will go bad ... If at the end a decision is made and feel like I had no choice or my ego won't allow it ... well that's another potential concern. Again disagreement isn't often itself the real problem. The real problems are: what if I know I can disagree but I don't? Lack of courage my side or unsafe environment made by management? If management tells me they've made their decision contradicting my input which applies: I feel listened to and feel like my input was taken seriously? That's good. Or perhaps I'm insecure and can't deal with loosing control. If I don't have self awareness here I might blame, critisize management or play victim. That can be bad. Maybe I just can't admit my assessment was incomplete or wrong. Also likely to hit into problems. And in any of this I refuse to change, I can sink the team in the limit.

I agree commiting to something silly whilst your mind constantly sees a better answer can lead to eventual breakup. Companies dont succeed by getting 100% buy in to bad decisions. People who see the writing on the wall frequently leave, leaving committed people to their Titanic or hindenburg.

Buy in resolves people with wrong ideas trying to steer into the iceberg, and loses the benefit of disagreement overcoming resistance and inertia towards bad goals.

My opinion is that committing is buy-in. You don't have to agree with something to buy into it. You buy into something when you agree to do it, whether or not you are convinced it's the right thing.

so "committing to and supporting that decision" isn't buy-in? Come on. That's spending your capital as an employee on the decision, buying into it and working on it. You're not harboring resentments

I can definitely commit to something that I disagree with. Ask any military person that is responsible for both making and accepting decisions. This may be a nuanced language here, but buy-in for me is the Cambridge definition "the fact of agreeing with and accepting something that someone suggests". For commit and support, I do not have to agree.

Only in an environment where failure is an option and inaccurate or wrong action is preferable to inaction. Disagree and Commit doesn't prevent the Challenger from blowing up.

There is a big difference between "I disagree with { on separate lines" and "this won't work in production".