If interstellar travel ever becomes possible you'd already have access to all the resources of the solar system as well as the output of the entire sun.

The scale of the problem l, technologically simply renders earthbound resource constraints irrelevant.

Like you're into "synthesize antimatter with solar power" at that point.

> If interstellar travel ever becomes possible you'd already have access to all the resources of the solar system as well as the output of the entire sun.

I'm not going to argue circular conditions, this is precisely why we should preserve dense energy sources, first an alternative abundant energy source must be demonstrated, before squandering it locally.

> The scale of the problem l, technologically simply renders earthbound resource constraints irrelevant.

Hidden in such statements is the implicit assumption that mining the solar system for fissile materials is less energy intensive than mining them locally.

We should make sure interstellar travel remains affordable by the time we decide to afford seeding other star systems.

Nothing prevents interstellar travel with current technology, it would just take a long time. We should keep this mode of travel, where survival on the ship is powered with known feasible technology (nuclear fission) on the table and conserve fissile materials until we succeed in compact fusion plants, in that case this constraint no longer is an argument to preserve fissile materials.

Speculating other energy storage technology like "antimatter storage as a battery to store solar power" before launching to another star is just that: speculation. We shouldn't squander fissile materials on the basis of feel-good speculation.