TL/DR: The Schlieffen Plan wasn't a 'bad' strategy, other than that it didn't allow for command and control's failure to evolve as fast as the size of the armies increased. Alternative plans proposed after the war probably wouldn't have done any better, mostly for the same reason.
Recent scholarship has suggested there was never really such a thing as a 'Schlieffen Plan', at least not in the cut-and-dried form it's often perceived. Schlieffen implemented an 'aufmarsch' schema, a plan by which German armies would mobilise, deploy and react to the circumstances of the moment. His plan encompassed the grand strategic situation, and proposed a rapid attack on France, followed by a transfer of force eastwards to counter the Russians. Schlieffen's contribution was a plan to get boots on the ground, to get them moving, and to provide a series of alternatives based on the situation. Moltke took this, sent enormous forces through Belgium, and disrupted four of the five French armies in the field with superior numbers and firepower. Lanrezac's Fifth Army remained intact, but German attempts to eliminate it were compromised by the enormous armies involved and the limited command, communications and intelligence resources available. Essentially the same thing happened on the giant right wing sweeping through Northern France: huge forces, large distances, poor communications and underdeveloped logistics. A sweep west of Paris was never really a practical consideration; sustaining a cohesive front would have been very difficult, even if the extra units from the left wing had been present, simply because controlling them was beyond the machinery of command at the time. It's conceivable that Moltke could have won on the Marne if he had held firm, but it was an operational question. The confounding factors of distance, exhaustion, the sudden appearance of the French Sixth Army, and the sheer impossibility of managing such huge formations saw the Germans stopped in their tracks.
As for alternatives, I know of two proposed after the war. One simply called for a large operational reserve(1 or 2 corps) to be formed from the left wing, and sent in once the schwerpunkt had been identified. This might have been a game changer if they had been available in the right place at the right time, but even Germany's excellent rail system would have been stretched to move that many troops in time to react to, say, Maunoury or the BEF. Another plan proposed a prompt charge on Verdun by 6-12 divisions, marching unsupported to take the fortresses. Based on the idea that the Germans would mobilise first, it was immediately apparent it would have failed, since the French were in the field two days before the Germans.