Most Seniors are driving the roadmap simply by virtue of being a Senior, and most Juniors have no opportunity to influence the roadmap because Seniors control the decisions.

Of course ideally everyone can contribute, but I think that's relatively rare.

I how ever could already experience a more open scrum process in which we all have a say.

For me it all started in an university project with four (incl me) people. I denied a good portion of the requests my supervisor gave me there, not because they were bad, but because they simply did not fit into the time budget or were not realistic. She was actually very happy about that because she was not that tech savy back then and learned a lot threw the process. Very good experience for me and we finished the project on time and exceeded expectations.