A long range gun seems a lot more vulnerable to counter artillery strikes considering they'll likely have to be extensively stabilised and would definitely be much heavier than PHL-16 trucks
With China's extensive manufacturing it's probably easier to stockpile thousands of 300/370mm reload rounds than developing a new gun system
Counter artillery would have to be conducted by cruise missile by most adversaries.
In the case of Taiwan, they would only have SLAM-ER, giving the artillery truck about 30 mins to pack up and go
Pretty much the calculation of cost advantage is depending on the # of shells fired. Perhaps the kind of ammo expenditure they are seeing in Ukraine is leading to the current piqued interest.The gun imaged in that article, the m85 costed a whopping $800,000 to build 65 years ago, that kinda just illustrates how incredibly expensive this endeavour would be compared to a rocket launch systems which is comparitively more simple. you would be trading ongoing cost for upfront costs.
The fact that the US found it infeasible and overlapping with other projects might be a hint that the cost gap between guns and missile is not enough to warrant fielding an entire new system.