More I think about it, “sustainability” of reusable rockets mostly shifts the problem: launchers save costs, materials, supply chain factors ... but re-entry and delivery of more assets remain non-sustainable for everyone else (i.e., atmospheric impacts).
We jumped to 'could' without the 'should'.
Long-term, payload delivery systems should probably transition to non-rocket systems such as launch loops or skyhooks. Both however require substantial upfront cost and in the case of launch loops a constant stream of power. And both have dramatic failure modes. There are no perfect solutions alas.
I'm thinking more along the lines of regulatory changes.
E.g., does SpaceX need to launch 12,000 Starlink or can it do the job with 3,000 of them? That kind of framework could reduce environmental impacts.
I get we need to launch rockets (e.g. important Earth Obs) but regulation might help.
