For us, an accurate delivery date on a 6 month project was mandatory. CX needed it so they could start onboarding high priority customers. Marketing needed it so they could plan advertising collateral and make promises at conventions. Product needed it to understand what the Q3 roadmap should contain. Sales needed it to close deals. I was fortunate to work in a business where I respected the heads of these departments, which believe it or not, should be the norm.
The challenge wasn't estimation - it's quite doable to break a large project down into a series of sprints (basically a sprint / waterfall hybrid). Delays usually came from unexpected sources, like reacting to a must have interruption or critical bugs. Those you cannot estimate for, but you can collaborate on a solution. Trim features, push date, bring in extra help, or crunch. Whatever the decision, making sure to work with the other departments as colaborators was always beneficial.
The reluctance to accept the reality that it cannot be made true achieves nothing positive for anybody. Rather it results in energy being lost to heat that could otherwise be used for productive work.
This isn't about respect between functions, this isn't about what ought to be professionally acceptable in the hypothetical. It's about accepting and working downstream of a situation based in objective truth.
Believe me, I wish it were true that software estimates could be made reliable. Everyone does. It would make everything involved in making and selling software easier. But, unfortunately, it's not easy. That's why so few organisations succeed at it.
I don't present easy answers to the tensions that arise from working downstream of this reality. Yes, it's easier to make deals contingent on firm delivery dates when selling. Yes, it's easier to plan marketing to concrete launch dates. Yes, it's easier to plan ahead when you have reliable timeframes for how long things take.
But, again unfortunately that is simply not the reality we live in. It is not easy. Flexibility, forward planning and working to where the puck is going to be, and accepting redundancy, lost work, or whatever if it never arrives there is part of it.
That I think is what people in different functions are best served rallying and collaborating around. One team, who build, market and sell software with the understanding that reliable estimates are not possible. There simply is no other way.
If you can't make firm delivery commitments to customers then they'll find someone who can. Losing customers, or not signing them in the first place, is the most harmful thing to everyone in the organization. Some engineers are oddly reluctant to accept that reality.