A PAK 40 only cost 12,000 RM. A Panzer IV cost 103,000 RM. During the course of the war, Germany only made 20,000 PAK 40 guns, and that number was split between two fronts. What if they'd made 100,000 or 150,000? Considering that in 43/44 Germany found itself in a defensive war and the versatility of the gun itself - both anti-tank and anti-personnel rounds - they'd have been much more effective than say doubling the number of Panzer IVs or Panthers. Few towed guns are "very mobile" but at a cost of 12,000 RM they could afford to lose more of them than they could tanks or TDs and they could have produced far more at that cost. They only weighed 3000 pounds which doesn't take much of a vehicle to tow it away when the counterfire began zooming in. In some ways, the mistake mirrors the early American reliance on towed tank guns. A three inch gun was great for defense, but we were fighting an offensive war. The Germans were doing just the opposite - still concentrating on offensive weapons while fighting a defensive war on two fronts.