You're doing the Product Manager's job, in my opinion. They're the ones who have to justify to senior management why something is being done, and when. The devs really should just get on and do it once it's been agreed.
Nope, I'm not doing the PM's job. I'm not doing the market research or coming up with the product roadmap. I want the PM to justify to me the work they are asking me to do. I also want to have a productive conversation about the cost of what they are asking me to do (which I can articulate to some extent) and the value they expect it to return.
Involving devs after the roadmap has been decided is too late.
I'm curious as to know what your PM's roadmaps look like, because really they shouldn't be listing features/implementation, but objectives and goals.
I agree though with what you're saying, regardless of the technical capacity of the PM, it's not really up for him to decide the capability of the teams, or the proposed technical solution. He should be managing those with the engineering teams, and their thoughts should be reflected on the roadmap.
I would say that as the PM role is one without actual power, you need to convince those who will be working on a problem to be convinced that what you're doing makes sense. With data backing them up, this makes your assumptions more trusted, and the team easier to convince.
Involving devs after the roadmap has been decided is too late.