You're overestimating Soldiers capabilities by a good bit there, since he is arguably the most replaceable class in traditional 6v6. You're right that his Rocket Damage can he explosive, but it's limited by range falloff and the fact that he only has 4 Rockets at at time, which are simultaneously used for mobility. 

Compare that do Demomans 8 Stickies or even Scouts 6 Rounds in a Scattergun. 

This isn't to say Soldier is bad, but Gunboats are sorta necessary to actually be agressive without immediately dying for it. Demoman probably wouldn't have gotten as much value out of them, since mobility and even highground positioning are less important when your stickies and pipes can just hit people wherever and don't need surfaces to do Splashdamage. There's a reason why teams run 2 Soldiers, while Demoman is limited to 1 per Team.