The German 37mm ATG was more than sufficient to take out the Soviet T-26/T-26S, and performed competently against the BT-5, BT-7/BT-7A, and light T-50 and T-60 series (prior to the T-70). When the T-34 began showing up, the 37mm guns were incapable of dealing with them beyond ludicrously short ranges (100m), allowing Soviet infantry operating closely with the tanks to locate the AT guns. The "door knocker" moniker had already been well established by then, due to poor performance against French tanks.
The Soviets had built a large quantity of T-26 tanks in the late '20s and early '30s, which was mediocre when new (but the 45mm gun was decent), and had fallen into complete obsolescence by the time of Barbarossa, but was still incurring steep maintenance and repair costs. The BT series (mostly with the same gun) was intended to replace it, but only did so partially, and was also becoming a bit "over the hill" but at least "serviceable". The T-34 (with a heavier 76mm gun) was not being produced in volume yet due to the high expenses of the existing armored force, and was only seen as an "interim" design, until the German invasion suddenly turned its production into a top priority. If Barbarossa had been delayed, or the Soviets had made a preemptive attack on Germany in '43 or so, the T-34 would possibly have already been sidelined after a short run, in favor some even newer design that would have been mass-produced.
[ At one of the annual "WWII Weekend" shows at Reading Airport, PA, there was a 75mm German AT gun on display. A corner of the forward plate of the gun shield had been broken off, driving the pieces through the more flexible rear plate, and a couple of deep grooves down the side of the breech block gave mute testimony to the fate of the crew when the shrapnel reached them. ]