To keep it simple... you can't have one line that eats 100 people... being fed by multiple other lines feeding 100 people into it and not have the system cascade.
As an example of this, think of this worst case scenario and how it will play out.
EPCOT is still closed.
DHS is open early and Star Wars is open too.
All DRR guests boarding headed to DHS.
All POP/AOA guests boarding headed to DHS.
All of CBR walking to station, headed to DHS.
In this scenario, 2 lines will be dumping guests into the CBR hub, plus guests are coming through security into the CBR hub. All guests want to leave through the single line to DHS.
At this point, there is going to be a backup of guests and the queue to head out on the DHS line is going to grow. The good news is, the queue is just people standing around not gondolas stacking up somewhere.
To make the line feel less of an issue, and avoid having to many people standing there, they will probably slow down the POP/AOA and the CBR/EPCOT lines. This will limit how fast those 2 lines can dump people into the queue. The security screening at the station will limit how fast CBR guests can arrive. With the CBR/DHS line running at maximum speed to process as fast as possible.
If this was all gondolas making automatic transfers between lines, there would need to be a huge holding area for the gondolas stacking up waiting to go to DHS, as they take up more space than people standing around. ALSO, there would need to be a huge holding area for cars coming back from DHS to feed into the other lines so they don't run out of cars. The holding areas would need to be large enough to absorb and buffer long enough for all the lines to equal back out, otherwise it would all lock up and freeze. All much much more complicated than just having the people get off and walk to the new car, creating a disconnect between the lines. Just like people moving from the MK Monorail loop to the EPCOT Monorail loop disconnects how those 2 loops schedule trains.