With Marathon and Plantation, now it looks like they'll just need Maine Superchargers to from the Keys to Canadian border.
To be technical about it, you can already get from the Keys to the Canadian border at Detroit (more quickly than Maine will be) or Seattle and in a couple weeks via Buffalo.
To be real technical about it , without Marathon, you CANNOT get from Key West to Canada on superchargers. The distance from KW to the nearest SC in N Palm Beach is over 200 miles it would be a scary ride pushing the limits not one most would want to take.
To be very technical about this route, because of the relatively low speeds on US1 across the Keys, EV Trip Planner says that using standard parameters in an 85 on 19's, it only takes 220 rated miles to cover the 236 road miles. Starting in Key West with a 100%, overnight charge, this should be a relatively easy drive in an 85. Proof of ability does not mean there is good spacing. As have argued many times 133 or so miles, as a maximum distance between Superchargers, makes for easy, comfortable driving of a Model S using Superchargers. Tesla understands this well, and the distance between Plantation and Marathon is 135 miles. It will be very nice when both of these South Florida Superchargers are open.