One Bridge too far and parachutists too far from the bridge to exploit the element of surprise...
I have not yet seen the Youtube videos you mentioned so what I can say on the matter is mostly based on what information I gathered from a couple of books I have on my boards.
The plan by itself made sense in so far as it was to spare the allied troops the later difficult crossing of the Rhine and possibly put a quick end to the war in a "Blitzkrieg" kind of manner.
With the benefit of hindsight it is always easy to say that it could have never worked. However, there were warnings before the start of the operation already, that the planning was too optimistic not taking the numerous possibilities of delays and problems into account which ultimately led to the defeat at Arnhem. Altogether there were not even nearly as many problems as could have been (e.g. the capturing of several bridges before they could be destroyed).
At least to some degree I think the operation was meant (though he would have been the last to admit it) to satisfy the ego of Bernard Law Montgomery who was not exactly happy about the secondary role he had played since Dwight D. Eisenhower had been named supreme commander of the allied forces in Europe. "Monty" meant to have a spectacular success and this eagerness for glory is what made him discard the doubts not only of his subordinates (Frederick Browning, commander of the first allied airborne army had previously stated that he thought they were going "one bridge to far" (hence the title of the later book and movie)), but also of his superior Eisenhower. Warnings of the presence of the 9. SS Panzerdivision which had been sent to London by members of the Dutch RÈsistance were vehemently ignored by Montgomery who wouldn't have any changes done to his plan. The inflexibility of the plan in combination with the rather high aim (not impossible to achieve I think but very difficult) and the unwillingness to react to the known presence of significant forces in the area all contributed to the failure of the operation. Montgomery was certainly not the first commander whose ambitions caused a defeat, but he was in a position to walk away fairly unscathed.
It is remarkable and possibly exemplary for his ego that in his long book "A history of Warfare" Montgomery repeatedly points to the battle of El Alamein with such strange thoughts as pointing out that the battle started almost on the same day (23rd October 1942) as the battle of Trafalgar (21st October 1805) to which he compares his own victory though there is very little in common between the two battles. My impression is that he really hoped to be compared to Lord Nelson who is to this day really revered in England. Operation Market Garden is not even hinted at with a single word in the entire long book.
There is one anecdote according to which Alan Brooke, the chief of the British imperial general staff once remarked to King George VI. that Montgomery was "a very good soldier, but I think he is after my job."
"I thought," the king replied after a moment's reflection, "that he was after mine!"
