Here's my experience: I actually served on towed 155mm (M114) and 105mm (M101A1) gun sections for several months in combat. Work on a cannon section is strenuous to say the least. You have to manhandle the stupid thing into position over ground that is never level, you have to dig it in, including digging trail pits and recoil pits, dig the position in/fill sandbags, put up a camo net, manhandle the ammunition boxes and break out the ammunition and of course, run through the manual labor of serving and firing the damn thing. With the 105, we often hit firing rates of 500-1,000 rounds per tube per day during Operations Prairie and Hastings on the DMZ. That meant convoys of trucks to the ammo dumps, working parties to unload the ammo and then break the WWII-era ammo out of its boxes and then strip it out of its creosote tubes. Hours of backbreaking labor in the sweltering heat. Towed guns are comparatively light compared to SPs and therefore the smaller ones can be moved by helicopter, however, most of the time you move by towing with a truck. The gun has to be broken down and march ordered and then hitched to the truck. If you are under fire, you have the choice of trying to get all that done so you can recover the guns, or just get the heck out of Dodge and hope there's something left to recover when you get back. While you're towing the guns, you are slow, loaded with ammunition and crew and extremely vulnerable to ambushes. We lost a lot of people during our convoys.
After Vietnam, I was commissioned and spent 24 years as an artillery officer, including commanding a 24 gun 155mm artillery battalion. I applied many of the lessons I learned as an enlisted cannoneer as an officer, including taking into account the strenuous and hazardous work serving the guns. The current fixation with 155mm towed artillery was frustrating because the people at the top saw the M198s as an improvement to direct support artillery where I saw a 7 1/2 ton behemoth that took 12 minutes to emplace in emergency conditions. After I left active duty, spent 13 years designing next-generation artillery. I applied the lessons I learned as an enlisted and commissioned officer - and Mechanical Engineer - to designing lighter, fast, more effective and less labor-intensive artillery support.
SP artillery is evolving and we have the ability to make them more effective and more reliable. Given the strong probability that we will eventually face large conventional forces again, it would be an enormous mistake to neglect self propulsion and armor protection and crew assistance systems for artillery.
So why did you say that anything bigger than 8 inches was too big to tow?