> Partial reusability won't get the cost down to the ~$100/kg range.

I don't see why $100/kg is a particularly important threshold.

> Falcon 9 can get 15 tons to LEO for $45 million, and that's already the lowest price on the market. To hit $300/kg they would need to build a 2nd-stage, launch, recover (on a drone-ship) and refurbish for $4.5 million. That's just not going to happen.

Falcon 9 reusable is 20 tons to LEO, and the cost SpaceX charges is what customers are willing to pay, not their costs incurred. Before they had reusability, the cost to make a complete, expendable falcon 9 rocket was $50 million. The marginal launch cost for a falcon 9 was approximately ~$15 million in 2020 when they were doing 20% of the launches they're doing now. They are very likely already below $500/kg. Remember this is a system that wasn't initially designed with any level of reusability in mind. A more optimized design that shifted more of the cost to the booster and which was produced at the appropriate scale would almost certainly see a further lower cost.

Falcon 9 boosters can fly back to an onshore landing pad, and Starship has already demonstrated landing back on the launch tower itself.

> If Starship fails, we are not getting $300/kg orbital costs for 1-2 decades minimum.

That's a substantially shifted goalpost. Even if starship succeeds we're still probably more than a decade out from $300/kg orbital costs being a reality. Rockets take a long time to develop, and longer still to mature. SpaceX has been working on Starship for over a decade already. Of course if we assume every rocket currently under development gets cancelled and something new needs to be started from scratch, it will take at least one full rocket development cycle time to bear fruit. But OP was worried that if Starship fails it will cause a loss of faith such that no one even starts working on another attempt for decades.