Losing 2 out of 10 seats is quite significant - especially when one of the major complaints about the transportation system is having to stand. This is again, people dreaming without focusing on what the real world complaints about the existing system are.
As I said, the current design puts the 'walk through' problems on the platform instead of the train.. if a cabin space is full, the person walks down the platform instead of 'through' the train. They could only walk to the other side of a cabin anyways.. making such value limited.
And the space gained isn't all that effective because it's space at people's feet on both sides. People are far less efficient space-wise then stationary physical objects. People want personal space, and they 'expand' with their bags, etc. You'd probably only get one line of people down that center in your example.. so maybe 4-5 people. For a total gain of 3 people while making the system worse in other ways.
A bad idea
What you again are failing to realize is while you would only lose two seats you would ultimately increase the number of seated passengers which in turn would increase the amount of standing space. The idea of addressing the walkthrough "problem" on the platform is actually the real problem they have right now since they are using what was only designed to be an unload platform for a lower capacity train as the load platform. It only takes a small number of people to stop at a car to block all of the remaining train to all guests, this results in cast members having to continuously yell at guests to move down. You also claim that the complaint is not enough seating, if this were the case Disney would be working to increase seating. In actuality they stopped using the fold down seats years ago and just recently actually removed them completely to make more standing room. That's 8 seats per car they removed.