I don't really understand the point of replacing the existing two tracks with one, custom designed track.
You'd have to spend the time and money designing something from scratch, while working within the confines of an existing building. Unless the plan is to tear the whole thing down like Tokyo? I don't see Disney bothering with that.
And what kind of coaster would it even be? One big downward spiral like Disneyland? Another version of Cosmic Rewind?
In theory, the advantage of consolidating to a single track is that it lends itself to more efficient operations. Assuming everything else remains equal (capacity, popularity, etc.) it would roughly require half as much maintenance to serve the same number of guests, because you now have half as many load stations, lift hills, brakes, linear feet of track, vehicles, etc. to inspect and repair.
There would also be a significant operations savings, since the current version essentially requires enough CMs to operate two separate rides (since that’s what they functionally are). It wouldn’t be completely halved, because a lot of the general crowd control positions are shared between the tracks (greeter, LL merge, etc.), but it would be close.
That said, having a single track then puts a lot more pressure on that one track to perform at capacity throughout the day. They can’t close one side of the ride for maintenance or repairs and keep the other side running, and even having a single slow dispatch would have a bigger capacity impact, as each dispatch represents a larger group of riders than the current 6-person trains.
Additionally, DL’s Space Mountain is a notoriously high-stress attraction for CMs, with dispatch intervals as fast as 16 seconds when running at full capacity. While waiting to board, I’s not unusual to hear the alarm that the ride is starting “cascade” (back up due to slow loading), at which point a vehicle will be sent for a second trip without giving the guests and opportunity to unload, in order to try and clear the brake zones behind it. They have found a lot of ways to keep guests moving quickly at that attraction, but it requires everybody to pay attention in a way that WDW’s more tourist-focused crowds would likely struggle with. It seems like a lot of WDI’s recent loading area designs have had downright-leisurely load times (Rise, Tron, and especially Ratatouille), so it’s difficult to imagine a modern attraction require such rushed loading, or that WDW’s guests and CMs would be able to keep up.