In hindsight, of course, you are right but that isn't the way it was seen at SHAEF or either of the Army Groups at the time. In addition, the only way that Eisenhower could capture the Ruhr at this point was by an envelopment from both North and South. Before launching Market Garden, Eisenhower assured Montgomery that this latter thrust by Hodges would have sufficient logistic support:What is meant by a "limited objective operation", I don't know, but I have a feeling that establishing a bridgehead over the Rhine was going to be the end of it for a while. Did not have the logistical capabilities to support a large enough force to envelop the Ruhr from the north, and secure the lines of communications.
If you read this section from Ruppenthal again:“You have been concerned that HODGES’ army may not have adequate maintenance to continue its advance in conjunction with your own thrust towards the RUHR. The United States Communications Zone and General BRADLEY are responsible for this maintenance and I assure you that it will be adequate for the task assigned to the Army on your right." (13 Sep 44)
Doesn't this suggest that on 22 Sep (i.e. almost a week into Op Market Garden), Eisenhower considered that, without Antwerp, he had sufficient logistic resources to seize the Ruhr and accomplish the other objectives laid out in the SHAEF directive of 13 Sep 44:At a meeting with his principal staff officers and top commanders held on 22 September he [Eisenhower] took pains to make clear his desire that all concerned "differentiate clearly between the logistic requirements for attaining objectives covered by present directives, including the seizure of the Ruhr and breaching the Siegfried Line, and the requirements for the final drive on Berlin."
Montgomery admitted in his Memoirs that he made a "bad mistake" in thinking that 21 Army Group could capture the Ruhr and concurrently clear Antwerp.The general plan, already explained, is to push our forces forward to the RHINE, securing bridgeheads over the river, seize the RUHR and concentrate our forces in preparation for a final non-stop drive into GERMANY. While this is going on we must secure bases as follows: Northern Group of Armies must promptly secure the approaches to ANTWERP or ROTTERDAM so that one of these ports and the lines of communication radiating therefrom can give adequate maintenance to the Northern Group of Armies deep into the heart of GERMANY.
My main contention, is that he wasn't the only member of the Allied High Command to make this mistake at this time.