That's part of the problem. Generally when one takes a complex system and focuses in a narrow-minded approach toward optimizing just one aspect they end up blowing it on other aspects. For example, an equally well reasoned but precisely opposite argument to OTRAG is Big Dumb Booster concept, where rather than trying to mass-produce many small rockets, you make singular giant rockets because when you compare the economies of giant rockets to those of small rockets, the giant rockets usually win.
OTRAG has some good concepts, but again I think they went too far. Not only are they pushing their propellant costs way up - which to be fair, is by design, accepting the fact that propellant is only a very small fraction of total costs - but they're also pushing up every last part of the handling costs, which unfortunately is not so small of a fraction of the total costs. And they're incurring a lot of size-related costs - load capacity of the pad and tower, environmental impacts on the surrounding area, etc - without gaining the typical size-related economies of scale, as OTRAG's extreme size only yields proportionally small payloads. It has almost no potential to optimize costs further, as they're willfully making propellant a significant fraction of total costs and the design basically throws away any potential for economic reuse. And with numerous heavy steel stages and the first stages having to separate at low altitude due to the low performance, it's basically a bomber ;) And with all of those stages clustered together they're really putting themselves at risk for cascading failures - stage separations are one of the riskiest parts of rocketry as-is, and cluster elements can interact in unexpected ways even when you only have a few of them.
So no, I'm not a big OTRAG fan, I think the design goes too far. I think SpaceX hit the right balancing point in this regard - enough of a degree of mass production to keep production streamlined (dozens of tanks and hundreds of engines per year), but not so much that you have to have huge numbers of stages and crazy-low performance (aka crazy-huge mass). They did this sort of balancing act in a lot of regards. For example, in rocketry there's often been a conflict between structural tanks (which can bear all of the loads during launch) and balloon tanks (which rely on internal pressure not to collapse). Balloon tanks have much better performance (meaning that they save you a lot of mass and thrust requirement - aka money), but they're a pain when it comes to handling because you have to keep them pressurized at all times after construction, even during transport, and if you have to do repairs, it's expensive. SpaceX uses a sort of semi-balloon tank design - their tanks are strong enough unpressurized to hold themselves up, but not to bear the forces of launch - they require internal pressure for that. So you can transport and handle them without hassle, but they still get excellent payload fractions - to the point that that if they were to launch their first stages without upper stages or payloads on them, they'd nearly be SSTOs. And the design is of course aided by their use of aluminum-lithium alloy - which normally is expensive to work in a reliable manner (it doesn't take well to being melted), but the friction stir seam welding system they use is really near ideally suited for it.
Just like in life, rocketry is about balance. OTRAG is more Kerbal-ish ;)