Agreed on both counts, but my assertion was that they're decreasing in popularity, which I absolutely stand by. Towed guns are much like trains in the post-WWII era. Sure there are still many things shipped by trains and many new trains have since been built. They're not on their way out, but they're certainly not the powerhouses of industry that they were in WWII.
In WWII, towed guns were the way. After WWII, armies that were restructuring along US lines agreed that self-propelled mobile artillery was the way to go. Warsaw Pact forces didn't start heading that way until the 70s and even then it was only half-hearted by comparison.
I have to say that there were two factors that really spelled the end for most US High Speed Tractors besides self-propelled artillery. One was the fact that the really heavy artillery (8 inch howitzers and larger) were also going out of style in the missile age of the late 50s and early 60s. The other was the fact that their high speed was meant to keep pace with US mechanized units, where self-propelled guns were better-suited. In the other cases of towed artillery, you were just as sensible using a truck, which is why I think we didn't see them in service any more.
The USSR clung both to heavy guns and towed artillery support for mechanized units for far longer.