More Sturmgeschutzen

The German Sturmgeschutz III & IV, Marder, Hetzer & Jagdpanther wrought a fearsome number of destroyed Allied & Soviet tanks, being low profiled & ideal for defence. Their disadvantage is that not having a turret, they could not mix it with tanks at close range.
http://www.achtungpanzer.com/stug.htm

Being much cheaper & easier to build than a normal tank, WI the Germans had concentrated from 1943 onwards only in Sturmgeschutz production, instead of "wasting" it on Panthers, Tigers and King Tigers? They would have had triple or quadruple the number of AFVs of OTL: enough to stymie D Day & the Russian attacks?

Sturmgeschuetze (aka assault guns) were less handicaped against turreted tanks then they would be today, since back then a tank had to stop in order to shoot with any hope of hitting except for the closest or ranges.The stopped turn rate of a Assault gun wasn`t far slower than that of a turret. Tanks back then usually (exception T34) had a rear-end engine but frontal drive and had to accomodate a driveshaft which made those tanks pretty high by todays standard. A lot of Assault guns were a lot lower (Hetzer less than 2 meters) which helps with camouflage and you can armour the front better. Assault guns also usually had a heavier gun then the tanks their chassis was based upon. (e.g. Panther 75mm gun/Jagtpanther 88mm) So back then assault guns, especially when used correctly could be very effective. Even after the war the idea lived on for some time (STRV 103, Jagtpanzer Kanone, Scorpion Pak)
On the other hand the later assault weren`t much cheaper then tanks of similar size and tanks are more flexible. Germany would have been probably best served if they would have adopted the alternate model to the Panther, which was more closly based on the t34. They could have upgraded the cannon to the 88 mm, like the russians did with the T34/85 and produce that one in large numbers.