Durability has mainly to do with armor and HP. Calliope is has much more of both.
The time a rocket launcher is firing is not the only time it vulnerable and the reason why PF fires faster is because of it sorter range and its performance when firing max range.
It easy already very hard to kill it if it was firing with lower CD it would very little difference to ability to survive but it would be far better at getting wipes.
One can counter katiousha and land mattress with artillery and even a 222 will kill them. In order to kill a Calliope one needs medium tank to dive.
There is simply no comparison.
I don't think we'll come to an agreement here. I do agree with you that a Calliope with a shorter shooting cycle wouldn't increase its durability by a lot, because its main source of durability is hp + armor. But for the rocket artillery which is easily destroyed by counter fire or a dive in the length of its firing cycle means a lot for their durability. Its for example really hard to Vet a land mattress when Stuka is on the field, because it will be constantly dewcrewed by counter fire while shooting.
Not really you simply have to compare commander with other commanders. A commander that bring too much to the table in different areas (AI,AT) is not good for diversity of the game. A commander that Calliope and P47 loiter would be an obvious choice in most modes.
But so would be a Commander with Ranger and Pershing or a Commander with Ranger and Calliope (two existing commanders). Both obvious choices. USF has some blank spots in their roster, even a Calliope/AT loiter commander would still miss some spots (for example how to kill onmap artillery pieces).
Ranger AT power is superior that PF.
Super Bazooka on Ranger is only better versus low armor tanks (Ostwind or smaller), overall they are roughly equal in combat situations for some reasons:
1) Shrek penetrates every stock allied medium tank at any range automatically while
Super Bazooka only penetrates an Ostheer PZIV at long range (which is most realistic) for about 77% and an OKW PZIV for about 60%.
2) Damage and deflection damage of Super Bazooka is lower.
3) While shrek accuracy is slightly lower at long range (higher at med and close) PFs reach Vet2 way faster than Rangers and get a 40% accuracy bonus while Ranger only get a 25% bonus. After that PFs hit any tank of the size of a medium or higher automatically at any range while Ranger still have a chance to miss at long range.
4) It doesn't mean a lot that Shrek has a 2 second longer reload cycle since you only get one salvo at a tank in normal combat situations (unless vehicle pathfinding hates you).
In the end Triple Super Bazooka cost 80 munition more to upgrade and takes away one more regular weapon. I don't think they perform better than PFs. In the end it is quite comparable op.
As I said before: Double Shrek and triple Super Bazooka are both bad for the game.
As long as we understand each other it is not even necessary to agree.
I have nothing to add.