Holding the chokepoint of the Dnepr and Daugava/Western Dvina in front of Smolensk would have been quite difficult simply given the correlation of forces. The Red Army's Western Front, the major strategic grouping of forces facing German Army Group Centre on the Smolensk axis had been absolutely hammered in the border skirmishes of 22 June - 5 July. Quoting from Volume 1 of Glantz's Barbarossa Derailed:

By 30 June, despite [the commander of Western Front General] Pavlov’s frenetic measures, Hoth’s and Guderian’s panzer groups had closed their pincers around a huge pocket west of Minsk containing virtually all of Pavlov’s totally disorganized 10th, 3rd, 4th, and most of the reserve 13th Army. The Western Front virtually ceased to exist as an organized force...In eighteen days of combat, Bock’s Army Group Center advanced 600 kilometers to the approaches to the Western Dvina and Dnepr Rivers, occupied all of Belorussia, and inflicted 417, 790 casualties on the Western Front, including 341,073 soldiers killed, captured, or missing. In addition, the Western Front lost 4,799 tanks, many of which simply ran out of fuel, 9,427 guns and mortars, and 1,777 combat aircraft, most of these destroyed at their airfields on the ground. In essence, at least along the Western strategic axis, Army Group Center’s powerful juggernaut fulfilled the mission assigned to it by the Barbarossa Plan by utterly shattering and destroying much of the Red Army’s first strategic echelon west of the Dvina and Dnepr Rivers.

(Emphasis added; Ch. 1, pageless ebook, see notes 40-41)

By July 4th four armies from Marshal Budenny's strategic reserves had been transferred to the Western Front's first echelon, now under the command of Marshal Semyon Timoshenko, General Pavlov having been executed. A fifth reserve army was forming up right in front of Smolensk as well. Western Front's first echelon did indeed form up at this natural chokepoint, the so-called "Смоленские ворота" [Gates of Smolensk] before the land bridge between the two rivers. However, by this point the odds were not in their favor whatsoever. The Soviet reserves had problems deploying rapidly during the early days of the war, both because of persistent harassment by the Luftwaffe and because of structural issues. By July 4th, two days into Army Group Centre's offensive maneuvers towards the "Gates of Smolensk," only 24 of the 60 divisions in Timoshenko's first echelon were in place; this meant that the Germans had a numerical superiority of 1.5:1 in infantry, 1.7:1 in artillery, and 7:1 in armor. The subsequent Soviet commitment in part or in full of a half dozen additional mechanized corps did balance these numbers out somewhat, but not until after the assault had begun. Some of these mechanized reinforcements were fed haphazardly into a large but fruitless counterattack at Lepel', which delayed the German advance slightly at a steep cost. This would become a repeat pattern throughout the battles on the Smolensk axis.

The Red Army's penchant for premature counteroffensives in the early war would reach a particular crescendo once the battle for Smolensk proper was committed. Time and time again throughout late July and August, Timoshenko and the Stavka would commit forces to offensives against the Wehrmacht's flanks as well as the teeth of the assault- some more well-prepared than others. These all suffered from a lack of coordination between the elements, a lack of logistics support, and tactical immaturity on the part of junior commanders. The Luftwaffe's complete air dominance did not help matters; some formations estimated that over fifty percent of their casualties were from Luftwaffe strikes, and the constant threat of air interdiction exacerbated the Red Army's logistics, command and communication problems.

Now, could the Red Army have had better foresight? Certainly. But hindsight bias is strong here, and we know things about Hitler's intentions that would have been difficult for Soviet planners in June 1941 to know. We do know (as I outlined in another recent answer) that the Soviets did in fact deploy their forces in two deep strategic echelons, and that part of the disaster of the first couple days of the invasion was the fact that the forces of the first operational echelon were mostly in barracks rather than prepped and ready for an attack, rather than not being deployed far back enough. The real bugbear for Soviet forces during the initial period of the war was the disconnect between their doctrinal focus on prompt counteroffensives and the technical & tactical immaturity of their force. They were caught with their pants down in the midst of a massive effort to modernize, restructure, and most importantly train the force to be ready for the realities of modern mechanized warfare; these reforms would not have been complete for another year at least, and in the interim the Red Army was confused and caught on the wrong foot. The halfway-complete shift in force posture from the Stalin Line to the Molotov Line made things much worse. So, in my estimation, although hindsight is perfect, almost everything that could have gone wrong for the Red Army in June 1941 did go wrong.