I think every good experience I've had as part of a team, was because the business people, the designers, and the engineers were all respectful of each other, and had no power over one another. So I totally agree! Nothing stops that.
People always imagine every team has to be some little internal war, and I think the whole methodology-mill industry is built around that belief. People CAN'T compromise or work together, or at least we can't bet on that, so here's some process. That rubs me the wrong way. It's pretty patronizing, you're right that it's like treating engineers like children.
> Similar for sales or marketing. They can come to the engineers, asking them: "We would like to sell feature XYZ. Are we ready for that?" then the engineers might say: "Nope, ask again next year.",
This is a totally respectful conversation with understanding on both sides. I might want to just make sure that the engineer in this scenario would be open to:
Marketer: "Feature XYZ is going to make a massive impact if we could get it with some clients this year before competitor X. Even feature XY without Z would beat their offering. What do you think?"
Engineer: "Ah yeah, I can see how that would be a really good advantage to have. I think XY might still be a bit hard to hit for EoY with the quality requirements we have, do you know precisely what they're planning?"
Marketer: "They are really only shipping feature X for the first month I think. No screenshots of Y in their announcement. Being the first mover would be good, even without Y."
Engineer: "We can make X happen, easy."
That conversation is also very respectful and everyone is adding valuable knowledge. It feels more realistic to me as well. Sometimes "no" isn't a great answer when "could we try for something at least?" means we could solve a lot of problems/make a lot of money. That's sort of my ideal way of working now. I'll hold everyone to the expectation that they're good at what they do, and they respect that I'm good at what I do.