Capture every bridge on schedule and the highway still stops them cold. These were Waffen SS who weren't going to scare and scatter. Even if XXX Corps to Arnhem, they can't cross the river in any force to make the operation strategically successful. At best, its an enormous expenditure of resources for nothing more than closing up than closing up to more of the Rhine and liberating the SW Netherlands.
Read "It Never Snows in September" by Robert Kershaw. Actually the majority of the troops weren't SS, and even those that were, were often poorly trained and recently called to the ranks (the SS divisions had been destroyed and were being rebuilt). By that stage of the war the Germans were absolutely scraping the bottom of the barrel for manpower, so they threw into the Arnhem battle men with no training, "stomach" battalions, men from coastal batteries, just anybody who could be found. They had the same problem with equipment, so although a company of Tiger II's were used, so was a Panzer III which the found somewhere. This book goes a long way to explaining why the paratroopers were able to hold out as long as they did - some of the best troops in the British army fighting magnificently but often against poorly equipped raw recruits or second rate units.
The Germans couldn't understand why the British tanks stopped moving for the night when the road in front of them was open. There are so many what-ifs with this operation, surely it could have succeeded. What if the entire plan hadn't been revealed when a glider carrying all the details of the plan was discovered by the Germans? What if the radios had been tested in non-desert conditions? What if a landing closer to the final bridge had been tried? What if the advance to the bridge had been done in a different way - after all, one end of it was taken. What if the weather had been better. What if the Poles had arrived earlier. What if a an understrength panzer division wasn't there or its presence had been noted and allowed for (assuming the operation still went ahead). What if the arrangement of troops had been different, so that the Americans got the last bridge or lead the land attack. What if the last bridge had been captured but not one of the ones in between?