The port was useless since the Germans still controlled the approaches to the port.
Yes but they would eventually be removed - and emphasis on "Port captured intact" - a port that could handle 1000 ships a day as opposed to a port that has been heavily damaged - and emphasis on Dash - Brussels was captured on the 4th Sept - Antwerp on the 5th Sept (To put into context - the Falaise Pocket battle had ended on the 21st Aug)
Consider if you will that the original invasion plan called for Allied forces to be crossing the 'Seine River' at D-Day + 90 (About Sept 20th) - and here they are going about clearing Belgium -and moving up to the German Border - and in the case of Op Market Garden - trying to Cross into Germany
Op Market Garden was not directly responsible for clearing the estuary but it did make it difficult for units of 15th Army (Heer) to exist in the area between the Market Garden Salient and the Scheldt Estuary.
As it was it took some hard fighting to clear the land around Antwerp and Walcheren Island was always going to require a proper Sea-born assault with all the trimmings and I have no idea if that could have happened before it did?
So I would be surprised if the First Can and Second Br Armies could have cleared the area much faster than they did in order to open the Port
Note that XXX Corp and the Airborne Army was not operating in a vacuum - other 21st Army units were advancing across its frontage as well as Hodges with 1st US Army to the south. Note the Relative positions on the 15th Sept and then positions on 15th Dec on this map - the entire Allied line has hardly moved in that time - Allies outrunning their supplies and Stiffening German resistance.
map (URL for original as you can zoom in)
https://upload.wikimedia.org/wikipedia/commons/4/45/Ww2_map68.jpg