It would seem that way from a casual glance, but rocket piping is quite complex, and currently assembled from hundreds or thousands of segments. Being able to print such assemblies will permit faster, lighter production, and production of shapes currently infeasible to cast/assemble. It could be dramatic, or slight. The key will likely be implementing a scaling strategy and qc.
See, an R7 family launch vehicles - complete dinosaurs. Both complex, and lengthy to assemble, in addition to having monstrously complex launch sequence. What you say about that is totally true.
But there is a big BUT - being complex does not preclude them being produced like sausages with big enough manufacturing complex, big and competent labour force, and very good rigid procedures perfected over 60 years.
In comparison to the cost of the whole space program, launch vehicle costs are almost microscopic.
Every rocket is a project, not a product.
Every rocket launch is not a service, it is also effectively a project.
And it was an achievement of the crippled post-Soviet space industry in making launch vehicles and launches more close to being a product and a service. Prior to that, a commercial satellite launch was hard to do without spending an eternity doing talkshops with likes of Boeing, Lockheed or French government. When ex-Soviet rockets popped up on the market, procuring a satellite launch service turned, figuratively, into a grocery shopping.
I think the trend may well go in the other direction, and become more like shipbuilding. Due to reusability, we can afford to make costlier rockets, as they're useful for many launches. This may for instance mean the use of relatively expensive composite materials, such as in the BFR, which prioritises fuel cost above unit cost. Also, the efficiency of rockets generally increases with size.
Honestly, large-scale composites are actually cheaper per unit dry mass than metal 3D printed parts. Composites may be more finicky, but they're not necessarily more expensive in aerospace!
Carbon fiber composites are more expensive than stamped metal parts like automobile bodies, but if you're talking CNC machined or 3D printed metal parts, then carbon fiber can indeed be cheaper.
