Indirect fire can be stopped, or slowed, with point defense. Oddly enough it actually exists at the vehicle level as it should be, as opposed to using the equivalent of 5' guns in space. And armor that is equipped to resist direct hits from fusion weapons is going to be a tough-nut to crack with standard artillery.
When I was in the Army I worked the MLRS system. "Cheap" disposable rocket launcher systems seem an oxymoron to me, because 'cheap' and 'disposable' don't often fit into the same order of battle when it comes to sophisticated artillery. You still need all the same stuff you have with a normal artillery unit, except you aren't going to be reloading.
We could blanket a grid square with 12 rockets in about 60 seconds. The standard round was the bomblet, with each rocket containing hundreds. Anything dug in was safe, and tanks were more or less safe unless we managed to hit the 50gal drum on the back, or get very, very luck with an engine deck hit. All your gear on the outside was toast though... Anything else we could shred pretty easily. If you were going to be using laser designators the rounds are going to home in on the laser, so they'd hit in front of your tanks. Plus as soon as the round left the horizon counterbattery would pick it up and relay that info to any point defense nearby, and the tanks themselves. I suppose you could build a seeker that was programmable, but that kind of defeats the purpose of a seeker. It seems your asking for as much inaccuracy there as possible, and it would be better to have the artillery battery firing blindly at your coordinates. Which, by the way, would be pretty damn close with good crews and good gear.
As for the other discussion you guys are having, well, I don't have a dog in that fight. It's difficult sometimes to scale up today's tech to tomorrow's warfare because things can radically change, and sometimes they stay the same. I, personally, don't see tons of difference in warfare in the future of Traveller. People are higher tech, weapons are higher tech, but the basics of warfare haven't changed. The mechanics just seem to be different. Artillery still works the same (indirect or direct), tanks still would be tanks, and infantry are still the key to winning any battle. It's just that instead of rocks & dirks, catapults and cannons, they'll be using gauss rifles and plasma guns.