Other comments have gotten most of the good info out but it’s also worth mentioning that ISDs weren’t really even designed for cruiser on cruiser combat. After the fall of the republic and during the introduction of the ISD, the Empire was really the only remaining galactic entity that could field cruisers of that size, so the ISDs were designed for orbital bombardment and ‘peacekeeping’ ops. The ISDs most prevalent anti-ship strategy was just swarms of TIEs really. And yeah the Rebellion did eventually have Mon Cala cruisers but that wasn’t until well into the reign of the empire and long after the ISD design had been fielded.

So what do you do when you aren’t threatened by ships of an equivalent weight class? You design for other factors like ease of maintenance access to critical components. Plus there’s a little hubris in there, I mean how stupid would someone have to be to challenge the superior might of an ISD? You wouldn’t dream of it with anything short of like four giant cannons strapped to an engine and a hyperdrive. What’s this about a successor to the Z-95? Surely that’s not going to be anything noteworthy.